build_config: give the mrbc config a name of its own - #7199

build_config/mrbc.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=mrbc aimed a compiler only build,
carrying `mruby-bin-mrbc` and nothing else, at build/host, on top of
whatever a default build had left there.  No build/mrbc was ever
created.

Whether that costs anything is decided by the timestamps in build/host.
Run straight after a default build, rake finds every object it wants up
to date, compiles nothing, and the tree is untouched:

    $ md5sum build/host/bin/mrbc
    c6b0d6d168918de337fa27a8d5d2a262  build/host/bin/mrbc
    $ wc -l build/host/include/mruby/presym/table.h
    3161 build/host/include/mruby/presym/table.h
    $ MRUBY_CONFIG=mrbc rake -j8             # exit 0
    $ md5sum build/host/bin/mrbc
    c6b0d6d168918de337fa27a8d5d2a262  build/host/bin/mrbc
    $ wc -l build/host/include/mruby/presym/table.h
    3161 build/host/include/mruby/presym/table.h

With a core source edited since, it rebuilds, and what it rebuilds is
half the tree.  It exits 0 while doing it:

    $ touch include/mruby.h src/*.c
    $ MRUBY_CONFIG=mrbc rake -j8             # exit 0
    ...
          Config Name: host
     Output Directory: build/host
    $ md5sum build/host/bin/mrbc
    498ad7e2f0c3b9fc72708e5341af055d  build/host/bin/mrbc
    $ wc -l build/host/include/mruby/presym/table.h
    573 build/host/include/mruby/presym/table.h
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    9881d200e90c5a7df4a29a36b193fbab  build/host/bin/mruby
    8dbb8c26becf449f5871f9e090c8a4ff  build/host/lib/libmruby.a

build/host/bin/mruby and libmruby.a are the default build's, holding the
3161 symbol table, while the presym headers beside them now describe 573
and bin/mrbc is a compiler built against those.  `disable_libmruby`
keeps the archive out of the way, so nothing announces the change.

The next default build does not put it back.  It aborts, before it links
anything, on a symbol the smaller table does not carry:

    $ rake -j8
    include/mruby/presym.h:42:23: error: 'MRB_SYM__Rational' undeclared
    (first use in this function); did you mean 'MRB_SYM__generational'?
       42 | #define MRB_SYM(name) MRB_SYM__##name
    src/numeric.c:299:53: note: in expansion of macro 'MRB_SYM'
      299 |   if (mrb_integer_p(y) && mrb_class_defined_id(mrb,
          |   MRB_SYM(Rational))) {
    rake aborted!
    Tasks: TOP => build => ... => build/host/src/numeric.o

and it goes on aborting the same way on every later run, because the
presym list is what decides whether the headers get rewritten.
tasks/presym.rake declares it as `file presym.list_path => ppps`, and
the mrbc run rewrote build/host/presym along with the core .pi files it
scanned, leaving it newer than the gem .pi files a default build would
have to rescan to notice anything is missing:

    $ ls -l --time-style=+%H:%M:%S build/host/presym
    -rw-rw-r-- 1 ... 00:30:03 build/host/presym
    $ ls -lt --time-style=+%H:%M:%S build/host/src/vm.pi
    -rw-rw-r-- 1 ... 00:30:03 build/host/src/vm.pi
    $ ls -lt --time-style=+%H:%M:%S build/host/mrbgems/mruby-rational/gem_init.pi
    -rw-rw-r-- 1 ... 00:28:57 build/host/mrbgems/mruby-rational/gem_init.pi

The task is up to date, the 573 symbol headers stay, and every core
source naming a symbol outside them fails to compile.  Recovery is
rm -rf build/host plus a full rebuild.

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.
The rename is all this config needs: `build_mrbc_exec` supplies
mruby-bin-mrbc itself (:345), and `disable_libmruby` means no internal
mrbc build is created for it either way (:152).  It now builds into a
directory of its own and leaves an existing host build byte-identical,
through the same sequence that broke it:

    $ touch include/mruby.h src/*.c
    $ MRUBY_CONFIG=mrbc rake -j8             # exit 0
    ...
          Config Name: mrbc
     Output Directory: build/mrbc
    $ ls build/
    host  mrbc
    $ 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
    $ rake -j8                               # exit 0

The compiler it produces is the one it was producing before, byte for
byte, and it still compiles for the host build's mruby:

    $ md5sum build/mrbc/bin/mrbc
    498ad7e2f0c3b9fc72708e5341af055d  build/mrbc/bin/mrbc
    $ wc -l build/mrbc/include/mruby/presym/table.h
    573 build/mrbc/include/mruby/presym/table.h
    $ build/mrbc/bin/mrbc -o /tmp/t.mrb /tmp/t.rb
    $ build/host/bin/mruby -b /tmp/t.mrb
    ["1", "2", "3"]
    [:a]
    :sym

One thing changes for whoever runs this config.  bin/ symlinks are
installed for a `host?` build (:452), so on master this config also
pointed bin/mrbc at what it built.  A named build does not, and the
compiler is at build/mrbc/bin/mrbc instead.  Run from a clean tree it
now touches nothing else:

    $ MRUBY_CONFIG=mrbc rake -j8             # exit 0
    $ ls build/
    mrbc