[llvm] [RISCV] Use Zacas for AtomicRMWInst::Nand i32 and XLen. (PR #80119)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

Using amocas in preference to lr/sc when possible seems very sensible to me. LGTM.

https://github.com/llvm/llvm-project/pull/80119