Rollup merge of #145626 - folkertdev:prefetch-fallback, r=Amanieu

add a fallback implementation for the `prefetch_*` intrinsics

related ACP: rust-lang/libs-team#638

The fallback is to just ignore the arguments. That is a valid implementation because this intrinsic is just a hint.

I also added the `miri::intrinsic_fallback_is_spec` annotation, so that miri now supports these operations. A prefetch intrinsic call is valid on any pointer. (specifically LLVM guarantees this https://llvm.org/docs/LangRef.html#llvm-prefetch-intrinsic)

Next, I made the `LOCALITY` argument a const generic. That argument must be const (otherwise LLVM crashes), but that was not reflected in the type.

Finally, with these changes, the intrinsic can be safe and `const` (a prefetch at const evaluation time is just a no-op).

cc `@Amanieu`
r? `@RalfJung`

Author: jhpratt