Rollup merge of #141687 - RalfJung:atomic_compare_exchange, r=bjorn3

core: unstably expose atomic_compare_exchange so stdarch can use it

Due to rust-lang/stdarch#1655, cleaning up the atomic intrinsics will be a bunch of extra work: stdarch directly calls them [here](https://github.com/rust-lang/stdarch/blob/8764244589373b8b48864c0ad11fd9233c672249/crates/core_arch/src/x86_64/cmpxchg16b.rs#L58-L74).

Instead of duplicating that match, stdarch should use what we have in libcore, so let's expose that.

r? `@bjorn3`

Author: jhpratt