Auto merge of #146948 - folkertdev:hint-prefetch, r=Amanieu

add `core::hint::prefetch_{read, write}_{data, instruction}`

tracking issue: rust-lang/rust#146941
acp: rust-lang/libs-team#638

well, we don't expose `prefetch_write_instruction`, that one doesn't really make sense in practice.

The implementation is straightforward, the docs can probably use some tweaks. Especially for the instruction version it's a little awkward.

r? `@Amanieu`

Author: bors