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>

Must not be a bad idea to improve documents. [ci skip]

We have decided to go forward to 3.0 this year.

Devs do not love "3".  The only exception is RUBY3_KEYWORDS in parse.y,
which seems unrelated to our interests.

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

Split ruby.h

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66555 b2dd03c8-39d4-4d8f-98ff-823fe69b080e