Auto merge of #122520 - scottmcm:stabilize_unchecked_math_basics, r=jhpratt

Stabilize `unchecked_{add,sub,mul}`

Tracking issue: #85122

I think we might as well just stabilize these basic three.  They're the ones that have `nuw`/`nsw` flags in LLVM.

Notably, this doesn't include the potentially-more-complex or -more-situational things like `unchecked_neg` or `unchecked_shr` that are under different feature flags.

To quote Ralf rust-lang/rust#85122 (comment),

> Are there any objections to stabilizing at least `unchecked_{add,sub,mul}`? For those there shouldn't be any surprises about what their safety requirements are.

*Semantially* these are [already available on stable, even in `const`, via](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=bdb1ff889b61950897f1e9f56d0c9a36) `checked_*`+`unreachable_unchecked`.  So IMHO we might as well just let people write them directly, rather than try to go through a `let Some(x) = x.checked_add(y) else { unsafe { hint::unreachable_unchecked() }};` dance.

I added additional text to each method to attempt to better describe the behaviour and encourage `wrapping_*` instead.

r? rust-lang/libs-api

Author: bors