[llvm] [RISCV][MC] Emit `x8` as `fp` instead of `s0` (PR #135500)

[Sam Elliott via llvm-commits]

https://github.com/lenary commented:

This seems reasonable, as it aids understanding.

Please can you add an MC-layer test in `llvm/test/MC/RISCV`, including the disassembler?

https://github.com/llvm/llvm-project/pull/135500