[clang] [llvm] [RISCV][LLVM] Enable atomics for 'Zalrsc' (PR #163672)

[Sam Elliott via cfe-commits]

lenary wrote:

Is there an abridged history of the RISC-V "A" spec anywhere? I feel reasonably out-of-touch with how it has developed (and been split into sub-extensions). 

https://github.com/llvm/llvm-project/pull/163672