ABI-required target features: warn when they are missing in base CPU
Part of https://github.com/rust-lang/rust/pull/135408:
instead of adding ABI-required features to the target we build for LLVM, check that they are already there. Crucially we check this after applying `-Ctarget-cpu` and `-Ctarget-feature`, by reading `sess.unstable_target_features`. This means we can tweak the ABI target feature check without changing the behavior for any existing user; they will get warnings but the target features behave as before.
The test changes here show that we are un-doing the "add all required target features" part. Without the full #135408, there is no way to take a way an ABI-required target feature with `-Ctarget-cpu`, so we cannot yet test that part.
Cc ``@workingjubilee``
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
7e123e4940