Use stable_abi/compiled.{c,rs} on non-CRuby

[eregon]

Hello,

I'm coming from oracle/truffleruby#3396, in short we're trying to make Rust extensions work on TruffleRuby.
There were some minor fixes in RbConfig, etc which have been fixed in truffleruby-dev.

The remaining errors are in oracle/truffleruby#3396 (comment)
So for example:

error[E0609]: no field `flags` on type `uncategorized::RBasic`
  --> /home/eregon/.rubies/truffleruby-dev/lib/gems/gems/commonmarker-1.1.5/ext/commonmarker/.rb-sys/stable/cargo/registry/src/index.crates.io-6f17d22bba15001f/rb-sys-0.9.99/src/stable_api/ruby_3_2.rs:19:35
   |
19 |         let flags = rstring.basic.flags;
   |                                   ^^^^^ unknown field

This is because TruffleRuby doesn't and cannot expose flags on struct RBasic.
And anyway we can see the code in ruby_3_2.rs is highly CRuby-specific:

rb-sys/crates/rb-sys/src/stable_api/ruby_3_2.rs

Lines 19 to 26 in 991df6d

	let flags = rstring.basic.flags;
	let is_heap = (flags & crate::ruby_rstring_flags::RSTRING_NOEMBED as VALUE) != 0;
	if !is_heap {
	rstring.as_.embed.len as _
	} else {
	rstring.as_.heap.len
	}

So such code in this file should only be used on CRuby, and on no other Ruby implementation.
And https://github.com/oxidize-rb/rb-sys/blob/991df6de21d244631e1b889070a8e4136f4220fe/crates/rb-sys/src/stable_api/compiled.c should be used instead.
A good way to test if it's CRuby is RUBY_ENGINE == "ruby".

Could you fix this issue?

Related: #229 cc @ianks