[clang] [llvm] [RISCV] Add experimental support of Zaamo and Zalrsc (PR #77424)

[Alex Bradbury via cfe-commits]

asb wrote:

CC @jyknight who can hopefully confirm if my interpretation is correct. Based on your comment on #77814, perhaps the expectation is that libatomic would have a lock-free implementation for any __sync calls used up to the max atomic width (e.g. lr/sc based if there's no AMO instructions)?

https://github.com/llvm/llvm-project/pull/77424