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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

lr/sc builtins are extremely fragile: there's no reasonable way for the compiler to guarantee that the sc is placed in such a way that it will eventually succeed.  (The equivalent intrinsics do exist on ARM, but ARM has significantly stronger guarantees here.  Even then, it's not completely reliable.)

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