[clang] [llvm] [RISCV] Add MC layer support for XSfmm*. (PR #133031)

[Philip Reames via cfe-commits]

https://github.com/preames commented:

First pass of technical comments.  I need to take a much more careful look at e.g. encodings, but will do that on the next round.

https://github.com/llvm/llvm-project/pull/133031