Add a test ensuring all our targets set the features required by their ABI.

[RalfJung]

With #136147, we emit a warning (and hopefully eventually a hard error) when a target does not enable all the target features required by the declared ABI, or when it enables a target feature incompatible with the declared ABI. Ideally, we'd ensure that all built-in targets pass this test, but that is non-trivial: we need to actually generate an LLVM TargetMachine for the target and ask LLVM about the enabled target features and then check that against our ABI compatibility list. I'm not sure how to best write such a test.

It could be a ui test compiling an empty program, with a revision for each target, but that seems annoying... I'd prefer something that iterates over all targets automatically, rather than having to list all targets in some ui test.

Cc @workingjubilee @rust-lang/wg-llvm

[nikic]

We already have a tests for basic assembly support for all targets in https://github.com/rust-lang/rust/tree/master/tests/assembly/targets -- does that implicitly cover this check as well?

[RalfJung]

I guess it will once these warnings become hard errors. Currently I presume warnings would be ignored for that test?

[nikic]

Possibly, I don't actually know what happens to warnings in assembly/codegen tests, I guess they just get ignored. We could slap a deny on it though :)

[RalfJung]

This is a emit_warn, not a lint, so I actually don't know how to make it a hard error just for a particular file.