[llvm] [MC][RISCV] Add assembly syntax highlighting for RISCV (PR #65853)

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere approved this pull request.

LGTM. Might be worth having someone with more RISC-V ISA knowledge to sign off on this too before merging. 

https://github.com/llvm/llvm-project/pull/65853