Auto merge of #121223 - RalfJung:simd-intrinsics, r=Amanieu

intrinsics::simd: add missing functions, avoid UB-triggering fast-math

Turns out stdarch declares a bunch more SIMD intrinsics that are still missing from libcore.
I hope I got the docs and in particular the safety requirements right for these "unordered" and "nanless" intrinsics.

Many of these are unused even in stdarch, but they are implemented in the codegen backend, so we may as well list them here.

r? `@Amanieu`
Cc `@calebzulawski` `@workingjubilee`

Author: bors