Tracking issue for RFC 2574, "SIMD vectors in FFI"

[Centril]

This is a tracking issue for the RFC "SIMD vectors in FFI" (rust-lang/rfcs#2574).
The feature gate is #![feature(simd_ffi)].

Steps:

• Implement the RFC (cc @rkruppe @gnzlbg); WIP in [WIP] Implement RFC2574 for FFI declarations #59238 and Implement RFC 2574, "SIMD vectors in FFI" #86546.
• Adjust documentation (see instructions on rustc-guide)
• Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

• Should it be possible to use, e.g., __m128 on C FFI when the avx feature is enabled? Does that change the calling convention and make doing so unsafe ? We could extend this RFC to also require that to use certain types certain features must be disabled.