in child processes.

`make test-spec SPECOPTS="spec/ruby/command_line/rubyopt_spec.rb -T --yjit"`
fails because `ruby_exe` doesn't seem to propagate options to child processes.

[Feature #21109]

By always freezing when setting the global rb_rs variable, we can ensure
it is not modified and can be accessed from a ractor.

We're also making sure it's an instance of String and does not have any
instance variables.

Of course, if $/ is changed at runtime, it may cause surprising behavior
but doing so is deprecated already anyway.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

This will add +MOD_GC to the version string and Ruby description when
Ruby is compiled with shared gc support.

When shared GC support is compiled in and a GC module has been loaded
using RUBY_GC_LIBRARY, the version string will include the name of
the currently active GC as reported by the rb_gc_active_gc_name function
in the form

+MOD_GC[gc_name]

[Feature #20794]

[Feature #20205]

The warning now suggests running with --debug-frozen-string-literal:

```
test.rb:3: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

When using --debug-frozen-string-literal, the location where the string
was created is shown:

```
test.rb:3: warning: literal string will be frozen in the future
test.rb:1: info: the string was created here
```

When resurrecting strings and debug mode is not enabled, the overhead is a simple FL_TEST_RAW.
When mutating chilled strings and deprecation warnings are not enabled,
the overhead is a simple warning category enabled check.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>
Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

They were initially made frozen to avoid false positives for cases such
as:

    str = str.dup if str.frozen?

But this may cause bugs and is generally confusing for users.

[Feature #20205]

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>