use the unstable carryless_mul in the pclmul implementation

[folkertdev]

Eventually we should be able to use the intrinsics in stdarch so that miri no longer needs a custom implementation. Unfortunately the required optimizations didn't make it into LLVM 22, so we'll have to wait ~6 months.

[rustbot]

Thank you for contributing to Miri! A reviewer will take a look at your PR, typically within a week or two.
Please remember to not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it.

[folkertdev]

CI uses 2026-01-21/rustc-nightly-x86_64-unknown-linux-gnu.tar.xz for that bootstrap job, How regularly does that update?

[RalfJung]

It always the current rust nightly bootstrap compiler (the file storing that is here). If the job here fails, that means merging this PR would cause the next sync into the Rust repo to fail.

[RalfJung]

The rust bootstrap compiler always gets updated around a release, so the next update will be around Feb 27th.

It is a fundamental part of the rustc development process that new standard library features can only be used inside rustc/miri after the next bootstrap bump (when nightly becomes beta and then that beta becomes the bootstrap compiler).