[clang] [llvm] Add Smrnmi extension (PR #111668)

[Alex Bradbury via cfe-commits]

asb wrote:

Welcome to the project @dong-miao! I'm not sure why the auto-labeler didn't kick in to attach the backend:RISC-V label (this is used to help ping the right people), but I've added that now.

Just two quick comments as it's end of day here so I can't review properly right now:
* We try to make the PR title and description match the intended commit. For a something like this, we'd typically name it something like "[RISCV][MC] Add support for the Smrnmi extension"
* Please add a release note to the RISC-V section in llvm/docs/ReleaseNotes.md. e.g. "Support was added for the Smrnmi extension."

https://github.com/llvm/llvm-project/pull/111668