Mingw crt-git 12.0.0.r369.g0d4221712-1 now prohibits "command line
contains characters that are not supported in the active code page".
https://sourceforge.net/p/mingw-w64/mingw-w64/ci/0d42217123d3aec0341b79f6d959c76e09648a1e/

Already Ruby builds `argv` in `rb_w32_sysinit`, instead of mswin- or
mingw-made `argv`.  Just bypass the conversion in mingw crt.

Mingw crt-git 12.0.0.r369.g0d4221712-1 now prohibits "command line
contains characters that are not supported in the active code page".
https://sourceforge.net/p/mingw-w64/mingw-w64/ci/0d42217123d3aec0341b79f6d959c76e09648a1e/

Provisionally exclude tests that fail by passing such characters.

This patch introduce M:N thread scheduler for Ractor system.

In general, M:N thread scheduler employs N native threads (OS threads)
to manage M user-level threads (Ruby threads in this case).
On the Ruby interpreter, 1 native thread is provided for 1 Ractor
and all Ruby threads are managed by the native thread.

From Ruby 1.9, the interpreter uses 1:1 thread scheduler which means
1 Ruby thread has 1 native thread. M:N scheduler change this strategy.

Because of compatibility issue (and stableness issue of the implementation)
main Ractor doesn't use M:N scheduler on default. On the other words,
threads on the main Ractor will be managed with 1:1 thread scheduler.

There are additional settings by environment variables:

`RUBY_MN_THREADS=1` enables M:N thread scheduler on the main ractor.
Note that non-main ractors use the M:N scheduler without this
configuration. With this configuration, single ractor applications
run threads on M:1 thread scheduler (green threads, user-level threads).

`RUBY_MAX_CPU=n` specifies maximum number of native threads for
M:N scheduler (default: 8).

This patch will be reverted soon if non-easy issues are found.

[Bug #19842]

Chkbuild runs for each directories just under `test` directory when
`test-all` failed.  `test/excludes` itself should not be a target.