build_config: give the clang-asan config a name of its own - #7197

build_config/clang-asan.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=clang-asan wrote its
`-fsanitize=address,undefined` objects straight into build/host, on top
of whatever a default build had left there.  No build/clang-asan was
ever created.

Starting from a good default build, and exiting 0 while doing it:

    $ ls build/
    host
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    9881d200e90c5a7df4a29a36b193fbab  build/host/bin/mruby
    8dbb8c26becf449f5871f9e090c8a4ff  build/host/lib/libmruby.a
    $ ar t build/host/lib/libmruby.a | sort -u | wc -l
    99
    $ MRUBY_CONFIG=clang-asan rake -j8       # exit 0
    ...
          Config Name: host
     Output Directory: build/host
    $ ls build/
    host
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    07a8c1484756e079a731e8af15944606  build/host/bin/mruby
    5160fcdf4b7e757f4ac6f547fa7100c2  build/host/lib/libmruby.a
    $ nm -D build/host/bin/mruby | grep -c asan
    174

What is left in build/host is a sanitizer build at `-O0`, and the repo
bin/ symlinks point into it, because `host?` builds install them (:452).

The damage outlives the run, and here it does not stop at a tree that
measures the wrong thing.  libmruby.a is archived with `ar rs`
(lib/mruby/build/command.rb:258); r replaces by member name, and an
archive records only the base name, so it cannot hold two objects called
gem_init.o apart.  A default build's archive holds 155 members under 99
distinct names, 43 of them gem_init.o.  After the clang-asan run it
holds 173 under 107, 51 of them gem_init.o, since full-core.gembox
carries gems that default.gembox does not.  The next default build hands
`ar rs` its own 43, which cannot name the 8 that came from those gems,
and they stay compiled with `-fsanitize`.

full-core.gembox skips mruby-bin-debugger while default.gembox carries
it, so the default build's mrdb link is the one that reaches those
members, with plain gcc and no sanitizer runtime.  It aborts, and goes
on aborting on every later run:

    $ rake -j8
    ...
    ld: build/host/lib/libmruby.a(gem_init.o): in function
    `asan.module_ctor':
    gem_init.c:(.text.asan.module_ctor[asan.module_ctor]+0x5):
    undefined reference to `__asan_init'
    collect2: error: ld returned 1 exit status
    rake aborted!
    Tasks: TOP => build => bin/mrdb => build/host/bin/mrdb

Recovery is rm -rf build/host plus a full rebuild.

Name the build after its config, which is what build_config/asan.rb
already does, and its own comment says why.  full-core.gembox supplies
mruby-bin-mrbc through its glob, so the rename is all this config needs.
It now builds into a directory of its own and leaves an existing host
build byte-identical:

    $ MRUBY_CONFIG=clang-asan rake -j8       # exit 0
    ...
          Config Name: clang-asan
     Output Directory: build/clang-asan
    $ ls build/
    clang-asan  host
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    9881d200e90c5a7df4a29a36b193fbab  build/host/bin/mruby
    8dbb8c26becf449f5871f9e090c8a4ff  build/host/lib/libmruby.a
    $ ar t build/host/lib/libmruby.a | sort -u | wc -l
    99
    $ nm -D build/clang-asan/bin/mruby | grep -c asan
    174
    $ rake -j8                               # exit 0

The build it produces is the one it was producing before, and it is
green:

    $ MRUBY_CONFIG=clang-asan rake -m test   # exit 0
    bintest - Command Binary Test
      Total: 79    OK: 79    KO: 0    Crash: 0    Skip: 0
    mrbtest - Embeddable Ruby Test
      Total: 2312  OK: 2309  KO: 0    Crash: 0    Skip: 3

One symlink does not follow the rename.  mruby-bin-config installs its
product through `define_installer` rather than
`define_installer_if_needed` (mrbgems/mruby-bin-config/mrbgem.rake:31),
so bin/mruby-config follows whichever non-cross build ran last, named or
not.  That is what master already does for every named build carrying
that gem, and it is unchanged here:

    $ MRUBY_CONFIG=bench rake -j8
    $ ls -l bin/mruby-config
    bin/mruby-config -> ../build/bench/bin/mruby-config