build_config: give each host build config a name of its own - #7195

build_config/host-nofloat.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=host-nofloat wrote
`MRB_NO_FLOAT` objects straight into build/host, on top of whatever a
default build had left there.  No build/host-nofloat was ever created.

Starting from a good default build:

    $ ls build/
    host
    $ md5sum build/host/bin/mruby build/host/bin/mrbc
    c5ab48eae68084c5cbf07fccaa5cf0db  build/host/bin/mruby
    2138e925f0e738d9240e0e71d82a3a73  build/host/bin/mrbc
    $ MRUBY_CONFIG=host-nofloat rake -j8     # exit 0
    ...
          Config Name: host
     Output Directory: build/host
    $ ls build/
    host
    $ md5sum build/host/bin/mruby build/host/bin/mrbc
    f69af2ed9fd24b7ddcfb013114b8db8e  build/host/bin/mruby
    2138e925f0e738d9240e0e71d82a3a73  build/host/bin/mrbc
    $ build/host/bin/mruby -e 'p 1'
    trace (most recent call last):
    	[1] mrblib/kernel.rb:27
    mrblib/kernel.rb:27:in module_function: wrong argument type Symbol
    (expected Symbol) (TypeError)

That last failure is a boot failure and not a float one, this config
fails on any input both before and after this commit (see the end of
this message).  What it shows here is only that build/host/bin/mruby
has stopped being the default build's mruby.

build/host/bin/mrbc was left untouched while build/host/bin/mruby was
relinked, so the tree is not even a consistent `MRB_NO_FLOAT` build.
That mrbc still compiles a float literal afterwards:

    $ echo 'p 1.5' > /tmp/f.rb
    $ build/host/bin/mrbc -o /tmp/f.mrb /tmp/f.rb    # exit 0

The repo bin/ symlinks point into this tree as well, since `host?`
builds install them (:452), so bin/mruby is that binary too.  bin/mrbc
need not be the one beside it: tasks/bin.rake:2 installs the internal
build/host/mrbc/bin/mrbc over it for a `host?` build carrying no
mruby-bin-mrbc gem of its own, and that one is compiled with
`MRB_NO_FLOAT`.  Which of the two the symlink ends up at is decided by
which of them rake found out of date.

So what the tree hands back afterwards depends on what happened to get
relinked.  Any measurement taken through this config silently measures
a mixed tree, and recovery is rm -rf build/host plus a full rebuild.

Naming the build is not enough on its own.  The internal mrbc build is
created only for a `host?` build or one that carries mruby-bin-mrbc
(:153), and none of the gemboxes this config loads supplies it.  Only
default.gembox does:

    $ grep -n mruby-bin-mrbc mrbgems/stdlib.gembox mrbgems/stdlib-ext.gembox \
        mrbgems/stdlib-io.gembox mrbgems/metaprog.gembox mrbgems/default.gembox
    mrbgems/default.gembox:9:  conf.gem :core => "mruby-bin-mrbc"

so the rename alone aborts before it compiles anything:

    $ MRUBY_CONFIG=host-nofloat rake -j8
    rake aborted!
    external mrbc or mruby-bin-mrbc gem in current('host-nofloat') or
    'host' build is required
    lib/mruby/build.rb:364:in `mrbcfile'

Name the build 'host-nofloat' and add mruby-bin-mrbc so it can supply
its own compiler.  build_config/gctest.rb is the same pair for the same
reason, a named build with an explicit mruby-bin-mrbc line; asan.rb is
named too and picks mrbc up implicitly, through full-core.gembox.  The
config now builds into its own directory and leaves an existing host
build byte-identical:

    $ MRUBY_CONFIG=host-nofloat rake -j8
    ...
          Config Name: host-nofloat
     Output Directory: build/host-nofloat
    $ md5sum build/host/bin/mruby build/host/bin/mrbc
    c5ab48eae68084c5cbf07fccaa5cf0db  build/host/bin/mruby
    2138e925f0e738d9240e0e71d82a3a73  build/host/bin/mrbc
    $ build/host/bin/mruby -e 'p 0.1+0.2'
    0.30000000000000004

Whether the resulting `MRB_NO_FLOAT` binary runs is a separate matter
and is unchanged here.  It fails to boot the same way when built into a
directory of its own:

    $ build/host-nofloat/bin/mruby -e 'p 1'
    mrblib/kernel.rb:27:in module_function: wrong argument type Symbol
    (expected Symbol) (TypeError)

build_config/host-shared.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=host-shared aimed its
`-fPIC`, `enable_debug` objects at build/host and its libmruby.so at
build/host/lib.  No build/host-shared was ever created.

What that costs is decided by the timestamps in build/host, and both
answers are wrong.  Starting from a good default build, rake compares
timestamps rather than compiler flags, finds every object up to date,
recompiles nothing, and links the shared library out of the archive the
default build left, which is not `-fPIC`:

    $ ls build/host/lib/
    libmruby.a  libmruby.flags.mak
    $ MRUBY_CONFIG=host-shared rake -j8
    LD    build/host/lib/libmruby.so
    ld: build/host/lib/libmruby.a(codegen.o): relocation R_X86_64_PC32
    against symbol `stderr@@GLIBC_2.2.5' can not be used when making a
    shared object; recompile with -fPIC
    ld: final link failed: bad value
    rake aborted!

When the objects are out of date instead, the tree is rebuilt with
`-fPIC` and `MRB_DEBUG` into build/host and the .so is produced there,
and this one never announces itself afterwards.  Both configs load the
same gembox, so the object set matches, and the default build after it
finds the tree up to date, exits 0, changes nothing and hands that
foreign tree back.  The libmruby.so and libmruby.so.4.0 left in
build/host/lib outlive every later default build, because nothing in
the default pipeline knows about them.  Recovery is rm -rf build/host
plus a full rebuild.

Name the build 'host-shared'.  default.gembox supplies mruby-bin-mrbc,
so the rename is all the build itself needs, but the post-build pass
that adds the shared-library targets picks its target by name and has to
follow it, or the config would build and produce no .so at all.  The
header comment's build/host paths are updated to match.  The config now
builds into its own directory and leaves an existing host build
byte-identical:

    $ MRUBY_CONFIG=host-shared rake -j8
    ...
          Config Name: host-shared
     Output Directory: build/host-shared
    $ ls build/host-shared/lib/
    libmruby.a  libmruby.flags.mak  libmruby.so  libmruby.so.4.0
    $ objdump -p build/host-shared/lib/libmruby.so | grep SONAME
      SONAME               libmruby.so.4.0
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    c5ab48eae68084c5cbf07fccaa5cf0db  build/host/bin/mruby
    7156ead7b09d5deb56a44776fa23dc7a  build/host/lib/libmruby.a
    $ ls build/host/lib/
    libmruby.a  libmruby.flags.mak

build_config/asan.rb already does this, and its own comment says why.