rustc: add support for nontemporal_load intrinsic

[gwenn]

For stdarch, some intrinsics need this.
It should be similar to the nontemporal_store introduced by #45367.

[workingjubilee]

I have been informed that the required memory fencing to implement these is not supported by LLVM in a cross-platform way yet, so this is likely blocked on further codegen backend support in order to implement them safely, at least for now. https://lists.llvm.org/pipermail/llvm-dev/2020-April/141279.html

Though these may be technically usable but "just" really hideously unsafe and of dangerous soundness concerns! Unsure. :D Something to pay careful attention to.

[Mark-Simulacrum]

I'm going to close this - we have some intrinsics exposing this (e.g., _mm_stream_ps), and while there are some issues with this currently (#114582) I don't know that we expect to do more here. I doubt we'd want a stable cross-platform API for something this niche.