Add support for `// only-cfg-target-has-atomic` to compiletest

[jyn514]

Right now, you have to specify the platforms and architectures you want by hand, which makes it easy to leave them out or include platforms you shouldn't. It would be nice to automate this - maybe with rustc --print cfg?

See also https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/How.20can.20I.20ignore.20tests.20on.20platforms.20without.20atomics.3F.

Originally posted by @wesleywiser in #84039 (comment)

[RalfJung]

Can't you use #[cfg(target_has_atomic)] in the test to basically stub it out?

[jyn514]

@RalfJung no, because that will cause the test to fail (because it's missing the errors rustc would normally emit).

[RalfJung]

Ah, good point -- thanks!

[jieyouxu]

compiletest triage: this should probably use the needs-* machinery for capability requirements, i.e. //@ needs-atomic or a better name.