build_config: give the glib_hal_test config a name of its own - #7200

`MRuby::Gem::List#linker_attrs` drops every gem that builds a binary,
and hands the rest to each link of libmruby.a.  5c130e8 added that for
mruby#5210, where a gem's options reached binaries the gem had nothing to do
with.  It reads `bin?` as "not a library gem", and a gem can be both.

`build.libmruby_objs << @objs` (lib/mruby/gem.rb:103) is unconditional,
so a gem with a src/ or an mrblib/ puts objects in libmruby.a whatever
else it builds.  When such a gem also declares `spec.bins`, its objects
are in the archive and its linker options are not on the line that links
it, so anything but its own binary fails to link.

`mruby-task` is that gem as soon as `MRB_TASK_BUILD_DEMO` is set: the
define gives it `spec.bins` (mrbgems/mruby-task/mrbgem.rake:52), and it
carries src/ and a ports/<port>/ HAL whose glib port is what needs the
`search_package` libraries.  build_config/glib_hal_test.rb sets both, and
mrbtest cannot be linked there:

    $ MRUBY_CONFIG=glib_hal_test rake -m test
    ld: mrbgems/mruby-task/ports/glib/task_hal.c:132: undefined
    reference to `g_main_loop_quit'
    ld: mrbgems/mruby-task/ports/glib/task_hal.c:134: undefined
    reference to `g_thread_join'
    ...
    rake aborted!
    Tasks: TOP => bin/mrbtest => build/host/bin/mrbtest

    gcc -fsanitize=address,undefined -o "build/host/bin/mrbtest"
      .../driver.o .../vformat.o .../mrbtest.o .../mrbtest.a
      "build/host/lib/libmruby.a"  -lm

Drop a binary gem only when it contributes no object.  A gem that put
nothing in libmruby.a still has options that belong to its own binary
alone, which is what mruby#5210 asked for, and one that did needs them
wherever libmruby.a is linked:

    $ MRUBY_CONFIG=glib_hal_test rake -m test    # exit 0
    bintest - Command Binary Test
      Total: 0     OK: 0    KO: 0    Crash: 0    Skip: 0
    mrbtest - Embeddable Ruby Test
      Total: 832   OK: 831  KO: 0    Crash: 0    Skip: 1

    gcc -fsanitize=address,undefined -o "build/host/bin/mrbtest"
      .../driver.o .../vformat.o .../mrbtest.o .../mrbtest.a
      "build/host/lib/libmruby.a" -L/usr/lib/x86_64-linux-gnu -lglib-2.0
      -L/usr/lib/x86_64-linux-gnu -lgthread-2.0 -pthread -lglib-2.0 -lm

Those libraries appear twice because the gem calls `search_package`
twice, before and after this commit alike.

The second line follows from the first.  tasks/bin.rake:10 asks for
`linker_attrs(gem)` when it links a gem's own binary, and a gem that now
stays in the library set would be appended to it a second time, so its
options would be doubled on that line.  Append it only when it is not
already there.  build/host/bin/mruby_task_demo is linked with the same
line as before:

    gcc -fsanitize=address,undefined -o "build/host/bin/mruby_task_demo"
      .../mruby_task_demo.o "build/host/lib/libmruby.a"
      -L/usr/lib/x86_64-linux-gnu -lglib-2.0
      -L/usr/lib/x86_64-linux-gnu -lgthread-2.0 -pthread -lglib-2.0 -lm

Nothing in the standard gemboxes moves.  Every core binary gem ships a
tools/ and no src/ or mrblib/, so `@objs` is empty for all of them
(:62, :96, :97) and the set `linker_attrs` returns is the one it
returned before.  The default build links bin/mruby and bin/mirb with
byte-identical command lines, and `rake -m test` is green:

    $ rake -m test                               # exit 0
      Total: 2089  OK: 2041  KO: 0  Crash: 0  Skip: 48

The example from mruby#5210 behaves as it did.  A gem carrying only a
tools/foo/main.c and `spec.linker.library_paths << "/usr/lib/foo"`,
built beside mruby-bin-mirb, keeps that path off every line but its own,
and it appears once in the whole run:

    $ MRUBY_CONFIG=issue5210.rb rake --verbose > log    # exit 0
    $ grep -c /usr/lib/foo log
    1
    $ grep -o 'gcc.*bin/foo".*' log
    gcc -L/usr/lib/foo -o "build/issue5210/bin/foo" .../main.o
      "build/issue5210/lib/libmruby.a"  -lm
    $ grep -o 'gcc.*bin/mirb".*' log
    gcc  -o "build/issue5210/bin/mirb" .../mirb.o ...
      "build/issue5210/lib/libmruby.a"  -lm

build_config/glib_hal_test.rb opened an anonymous `MRuby::Build`, and an
unnamed build is called 'host' (lib/mruby/build.rb:94).  The build tree
is keyed by that name (:107), so MRUBY_CONFIG=glib_hal_test aimed its
objects at build/host, on top of whatever a default build had left
there.  No build/glib_hal_test was ever created.

This config loads no gembox at all, so what it shares build/host with is
a tree built from a different set of gems, and the presym scan cannot
tell them apart.  It reads the `.pi` files the compilers leave behind,
and those are keyed on the source timestamp alone, so a `.pi` a default
build preprocessed under its own defines is reused here unchanged.  The
result is a symbol table belonging to neither build, and this config
aborts on the first core file that needs a symbol it lost:

    $ rake -j8                               # a default build, exit 0
    $ MRUBY_CONFIG=glib_hal_test rake -j8
    include/mruby/presym.h:42:23: error: 'MRB_SYM__print' undeclared
    here (not in a function); did you mean 'MRB_SYM__private'?
       42 | #define MRB_SYM(name) MRB_SYM__##name
    src/kernel.c:811:29: note: in expansion of macro 'MRB_SYM'
      811 |   MRB_MT_ENTRY(mrb_print_m, MRB_SYM(print), MRB_ARGS_ANY() |
          |   MRB_MT_PRIVATE),
    rake aborted!
    Tasks: TOP => build => build/host/lib/libmruby.a
        => build/host/src/kernel.o

`MRB_SYM(p)` and `MRB_SYM(print)` sit behind `#ifndef HAVE_MRUBY_IO_GEM`
(src/kernel.c:809).  A default build carries mruby-io and its kernel.pi
does not name them; this build carries no io gem and its src/kernel.c
does.  The stale kernel.pi is what gets scanned, so the table is built
without them:

    $ md5sum build/host/src/kernel.pi        # before and after the run
    358985be208e175586d7b6486c9726b9  build/host/src/kernel.pi
    358985be208e175586d7b6486c9726b9  build/host/src/kernel.pi

The leak runs the other way too.  `MRB_SYM(Rational)` reaches the table
from the default build's numeric.pi, preprocessed with
`-DMRB_USE_RATIONAL`, which this build never sets.  Compared with the
same config on a clean tree, the list it writes is missing p, print and
the ten mrblib file names, and carries Rational that does not belong to
it: 442 symbols on a clean tree, 431 here.  The file names go missing
for the same reason, one step further along: `enable_debug` appends -g
to the mrbc options (:176), so this config's mrblib.c holds them and the
default build's, reused here, does not.

Whether it gets that far is decided by the timestamps in build/host, so
the config does build against a tree it does not clobber:

    $ rm -rf build/host
    $ MRUBY_CONFIG=glib_hal_test rake -j8    # exit 0
    $ ls build/host/bin/
    mruby_task_demo

which is the shape the tree is left in either way.  `MRB_TASK_BUILD_DEMO`
gives mruby-task a `spec.bins`, and bin/ symlinks are installed for a
`host?` build (:452), so the demo lands in the repo bin/ beside the
default build's commands, and build/host holds an `-fsanitize` tree at
-O0 with a 889 symbol table where the default build's has 3161.

Name the build after its config, which is what build_config/asan.rb and
build_config/gctest.rb already do, and what 8b44a52 did for host-cxx.
Naming is not enough on its own here.  The internal mrbc build is
created only for a `host?` build or one carrying mruby-bin-mrbc (:153),
and this config loads no gembox to supply it, so the rename alone stops
before it compiles anything:

    $ MRUBY_CONFIG=glib_hal_test rake -j8
    rake aborted!
    external mrbc or mruby-bin-mrbc gem in current('glib_hal_test') or
    'host' build is required
    tasks/mrblib.rake:9

build_config/gctest.rb is the same pair for the same reason, a named
build with an explicit mruby-bin-mrbc line.

mruby-bin-mrbc brings a bintest of its own, and the config already sets
`enable_bintest`, where before the rename it had no bintest to run at
all.  One of its asserts compares the disassembly mrbc writes against
the one `mruby -v -c` writes (mrbgems/mruby-bin-mrbc/bintest/mrbc.rb:85),
and nothing here supplies that binary, so it ends the run:

    Errno::ENOENT: mrbc -v disassembles like mruby -v
      => No such file or directory - build/glib_hal_test/bin/mruby
    Crash: 1

Add mruby-bin-mruby as well, so the test the compiler gem carries has
what it asks for.  With the three lines, the config builds into a
directory of its own, through the sequence that aborted:

    $ rake -j8                               # a default build, exit 0
    $ MRUBY_CONFIG=glib_hal_test rake -j8    # exit 0
    ...
          Config Name: glib_hal_test
     Output Directory: build/glib_hal_test
    $ ls build/
    glib_hal_test  host
    $ ls build/glib_hal_test/bin/
    mrbc  mruby  mruby_task_demo
    $ wc -l build/glib_hal_test/include/mruby/presym/table.h
    889 build/glib_hal_test/include/mruby/presym/table.h

and an existing host build is left byte-identical, with its own table
and its own bin/:

    $ md5sum build/host/bin/mruby build/host/bin/mrbc build/host/lib/libmruby.a
    9881d200e90c5a7df4a29a36b193fbab  build/host/bin/mruby
    c6b0d6d168918de337fa27a8d5d2a262  build/host/bin/mrbc
    8dbb8c26becf449f5871f9e090c8a4ff  build/host/lib/libmruby.a
    $ wc -l build/host/include/mruby/presym/table.h
    3161 build/host/include/mruby/presym/table.h
    $ ls bin/
    mirb  mrb  mrbc  mrdb  mruby  mruby-config  mruby-strip
    $ rake -j8                               # exit 0
    $ build/glib_hal_test/bin/mruby_task_demo
    [t=    0 ms] main: spawning T2 + T3; running T1 (glib-only) on main
    thread
    ...

`rake test` passes on top of the commit before this one, which keeps
the linker options of a gem whose objects are in libmruby.a.  Without
it, mrbtest cannot be linked here at all: `MRB_TASK_BUILD_DEMO` makes
mruby-task a bin gem, and `MRuby::Gem::List#linker_attrs` dropped the
`-lglib-2.0` that `search_package` contributed from the mrbtest link
line.  With the two together:

    $ MRUBY_CONFIG=glib_hal_test rake -m test    # exit 0
    bintest - Command Binary Test
      Total: 53    OK: 52   KO: 0    Crash: 0    Skip: 1
    mrbtest - Embeddable Ruby Test
      Total: 832   OK: 831  KO: 0    Crash: 0    Skip: 1
    $ ls bin/
    mirb  mrb  mrbc  mrdb  mruby  mruby-config  mruby-strip

A default build is unaffected, and `rake -m test` on it stays green:

    $ rake -m test                               # exit 0
      Total: 2089  OK: 2041  KO: 0  Crash: 0  Skip: 48