[clang] [llvm] [RISCV] Add riscv_atomic.h and Zawrs/Zalrsc builtins (PR #94578)

[Alex Bradbury via llvm-commits]

asb wrote:

My understanding from the sync-up call just now is you're planning to drop lr/sc intrinsics due to the concerns listed in this thread (which I share), but will keep pushing the zawrs intrinsics.

https://github.com/llvm/llvm-project/pull/94578