The fiddle gem is broken on truffleruby and jruby

[eregon]

Given https://bugs.ruby-lang.org/issues/20309 fiddle will become a bundled gem in Ruby 3.5+.

As a result people will add fiddle to their Gemfile where they previously wouldn't (and so there was no issue).
Unfortunately this leads to failure for example on ffi/ffi#1119:

• https://github.com/ffi/ffi/actions/runs/10881406447/job/30190278321?pr=1119 JRuby fails when trying to build the C extension
• TruffleRuby segfaults, probably because the fiddle C extension does something unexpected, and it's completely untested on TruffleRuby. It seems better to use the TruffleRuby Fiddle backend anyway for performance, as it avoids basically going twice through libffi.

I think the best solution for now is to just use fiddle from stdlib on non-CRuby. That's properly tested and working.
IOW, to not build or use the C extension on non-CRuby, similar to how it was done in ruby/stringio#71 or ruby/strscan#35

[eregon]

@kou Thank you for your help fixing this.
Could you make a fiddle release?
Then there should be no problem when gems/Gemfile's depend on fiddle for JRuby & TruffleRuby.

[kou]

Done.