build_config: give the cosmopolitan config a name of its own - #7245

build_config/cosmopolitan.rb opened an anonymous `MRuby::Build`, and an
unnamed build is called 'host' (lib/mruby/build.rb:114).  The build tree
is keyed by that name (:127), so MRUBY_CONFIG=cosmopolitan aimed a
cosmocc build at build/host, on top of whatever a default build had left
there.  No build/cosmopolitan was ever created.

Run after a default build, it does not get as far as a library.  The
`.flags` record beside each object notices the compiler changed and
rebuilds what the rules own, but the Prism objects are built by a `file`
task of their own (mrbgems/mruby-compiler/mrbgem.rake:105), which the
record does not cover, so the gcc objects stay and cosmocc refuses them
at the first link:

    $ rake -j16                              # exit 0, default build
    $ COSMO_ROOT=~/cosmo MRUBY_CONFIG=cosmopolitan rake -j16
    build/host: output here was built by another configuration, rebuilding it
      command added: /home/.../cosmo/bin/cosmocc
      command removed: gcc
    ...
    LD    build/host/mrbc/bin/mrbc.com
    cosmocc: fatal error: build/host/mrbc/mrbgems/mruby-compiler/lib/diagnostic.o:
    linker input missing concomitant
    build/host/mrbc/mrbgems/mruby-compiler/lib/.aarch64/diagnostic.o file
    rake aborted!
    Tasks: TOP => default => all => gensym => build/host/presym => ...
    => build/host/mrbc/bin/mrbc.com

By then it has preprocessed the core sources with cosmocc, so the next
default build finds its own tree foreign and does it over again.  It
exits 0 and what it links is byte for byte what it linked before, but
it preprocesses 198 sources and compiles 165 objects to get there:

    $ rake -j16 2>&1 | tee rake.log          # exit 0
    build/host/mrbc: output here was built by another configuration, rebuilding it
    build/host: output here was built by another configuration, rebuilding it
    ...
    $ grep -c '^CPP ' rake.log
    198
    $ grep -c '^CC ' rake.log
    165

Name the build after its config, which is what build_config/mrbc.rb,
build_config/clang-asan.rb and build_config/gctest.rb already do.  The
rename is all this config needs: it carries `mruby-bin-mrbc` itself, so
its internal mrbc build is created either way (:174).  It now builds
into a directory of its own and leaves an existing host build
byte-identical, through the same sequence that broke it:

    $ rake -j16                              # exit 0, default build
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    7a52da8807433dc3812c042d46e633fd  build/host/bin/mruby
    7492434aab8a47f72c1cbd48d1f7d093  build/host/lib/libmruby.a
    $ COSMO_ROOT=~/cosmo MRUBY_CONFIG=cosmopolitan rake -j16   # exit 0
    ...
          Config Name: cosmopolitan
     Output Directory: build/cosmopolitan
    $ ls build/
    cosmopolitan  host
    $ md5sum build/host/bin/mruby build/host/lib/libmruby.a
    7a52da8807433dc3812c042d46e633fd  build/host/bin/mruby
    7492434aab8a47f72c1cbd48d1f7d093  build/host/lib/libmruby.a
    $ rake -j16 | grep -c '^CC '            # exit 0
    0

The binaries it produces are the APE ones it was producing before:

    $ ls build/cosmopolitan/bin/*.com
    build/cosmopolitan/bin/mirb.com  build/cosmopolitan/bin/mrbc.com
    build/cosmopolitan/bin/mrdb.com  build/cosmopolitan/bin/mruby-strip.com
    build/cosmopolitan/bin/mruby.com
    $ build/cosmopolitan/bin/mruby.com -e 'p [1,2,3].map { |x| x.to_s }'
    ["1", "2", "3"]

and the command CI runs for this config passes from a clean tree,
touching nothing else:

    $ COSMO_ROOT=~/cosmo rake -m test:run:serial MRUBY_CONFIG=cosmopolitan
    $ ls build/
    cosmopolitan

One thing changes for whoever runs this config.  bin/ symlinks are
installed for a `host?` build (:482), so on master this config also
pointed bin/mruby and the rest at the .com files it built.  A named
build does not, and they are under build/cosmopolitan/bin/ instead; the
header comment that said bin/ now says so.