In test_bug_reporter and test_rubyoptions we intentionally
test child processes that cause SEGV. We run them with YJIT
if the parent uses YJIT so that the text description
matches the parent RUBY_DESCRIPTION.

If you run tests with RUN_OPTS=--mjit, the test fixes in
https://github.com/ruby/ruby/pull/5872 don't work.

Since enabling YJIT or MJIT drastically changes what could go wrong at
runtime, it's good to be front and center about whether they are enabled
when dumping a crash report. Previously, `RUBY_DESCRIPTION` and the
description printed when crashing can be different when a JIT is on.

Introduce a new internal data global, `rb_dynamic_description`, and set
it to be the same as `RUBY_DESCRIPTION` during initialization; use it
when crashing.

 * version.c: Init_ruby_description(): Initialize and use
       `rb_dynamic_description`.
 * error.c: Change crash reports to use `rb_dynamic_description`.
 * ruby.c: Call `Init_ruby_description()` earlier. Slightly more work
       for when we exit right after printing the description but that
       was deemed acceptable.
 * include/ruby/version.h: Talk about how JIT info is not in
      `ruby_description`.
 * test/-ext-/bug_reporter/test_bug_reporter.rb: Remove handling for
       crash description being different from `RUBY_DESCRIPTION`.
 * test/ruby/test_rubyoptions.rb: ditto

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Alan Wu <alanwu@ruby-lang.org>

Two tests abort intentionally and they create core files if
possible. In these case, we don't need to see core files
so disable by `"Process.setrlimit(Process::RLIMIT_CORE, 0)` for
those cases.

* Rename --jit to --mjit

[Feature #18349]

* Fix a few more --jit references

* Fix MJIT Actions

* More s/jit/mjit/ and re-introduce --disable-jit

* Update NEWS.md

* Fix test_bug_reporter_add

Since the YJIT Ruby module is CRuby specific and not meant for general
use, it should live under RubyVM instead of at top level.

There might be code out there that expect `ruby -v` to print only one
line. Since MJIT shows +JIT in `ruby -v` and RUBY_DESCRIPTION, let's
show +YJIT.

The crash report doesn't show anything about MJIT, so adjust the test.
The "test_ruby_version" test was unaware of RUBY_YJIT_ENABLE and so
was failing when the variable is set and inherited into the children
processes it spawns. Explicitly unset the variable in the test.