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

[Sam Elliott via cfe-commits]

lenary wrote:

Oh, sorry, I now see this was split and the MC layer was landed. I am going to mark this as closed, re-open if there was anything left in this PR that wasn't landed and is still relevant.

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