[clang] [llvm] [RISCV][MC] Add experimental `Zvvmm` MC support (PR #193956)

[Pengcheng Wang via cfe-commits]

https://github.com/wangpc-pp commented:

I noticed there are so many sub-extensions in the spec but it seems we only support `Zvmm` here?

https://github.com/llvm/llvm-project/pull/193956