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

[Jiuyang Liu via llvm-commits]

sequencer wrote:

> but won't merge them until they are ratified just like Zvzip/Zvabd/Zibimm

I think this patch is supported to be the vendor instruction set of `SiFive Xsfmm* Attached Matrix Extensions` if I understand correctly of this PR. 

AME is another issue that we don’t have ratified AME but just SiFive AME proposal, XuanTie AME proposal. 

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