[llvm] [RISCV] Add @earlyclobber to SiFive custom matrix multiply instruction. (PR #124060)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

I don't know this can be tested, but LGTM since this is a vendor extension.

https://github.com/llvm/llvm-project/pull/124060