avx512 support regression in 1.43

[lu-zero]

error[E0658]: use of unstable library feature 'stdsimd'
  --> src/cpu_features/x86.rs:50:12
   |
50 |         && is_x86_feature_detected!("avx512ifma")
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #27731 <https://github.com/rust-lang/rust/issues/27731> for more information
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

It was notified on zulip before

[shssoichiro]

Note that this worked in 1.42, so this is a stable-to-stable regression.

[Amanieu]

This basically #68905. We re-stabilized some of the avx512 features that were used by BLAKE3, but it seems that we missed rav1e.

[joshtriplett]

@Amanieu Any chance you could prepare a fix? I think this is worth a stable point release.

[joshtriplett]

Summarizing some discussions in the lang team meeting, regarding CPU feature naming: there are inconsistencies in feature naming between LLVM, GCC, GAS, and the Linux kernel, but we have a mild technical reason to defer to LLVM (doing otherwise requires renaming the feature). So let's defer to LLVM naming of CPU features, which doesn't tend to use underscores.