Unlike Clang 15, GCC 11 seems require default symbol visibility.

We have name fragmentation for this feature, including "shared GC",
"modular GC", and "external GC". This commit standardizes the feature
name to "modular GC" and the implementation to "GC library".

Warned if both of `main` and `wmain` are exposed:
```
LINK : warning LNK4067: ambiguous entry point; selected 'mainCRTStartup'
```

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.

This reverts commit 8ddb1110c283c5cb59b6582383f36fdbcc43ab19.

This commit changes the external GC to be loaded with the `--gc-library`
command line argument instead of the RUBY_GC_LIBRARY_PATH environment
variable because @nobu pointed out that loading binaries using environment
variables can pose a security risk.

* We always need use_sigaltstack=0 because Ruby registers sigaltstack
  handlers
* We also need to disable leak detection (unless RUBY_FREE_AT_EXIT is
  set - I might experiment later with automatically enabling leak
  detection if RUBY_FREE_AT_EXIT is set).

Burning it into the built ruby binary in this way avoids people needing
to remember to start their Ruby program with these flags all the time.

We also need a small fix in mkmf to make sure that test programs also
don't have leak detection enabled (this is never desirable)

[Bug #20256]

`ruby_set_debug_option` is needed for `RUBY_DEBUG_LOG` feature
so it should be called when `USE_RUBY_DEBUG_LOG` is true.