[clang] [llvm] [RISCV] Add MIPS extensions (PR #121394)

[Sam Elliott via llvm-commits]

https://github.com/lenary commented:

Some small notes, the one about using RISCVExtension will probably have the most knock-on work (run both the llvm and the clang test suites after that change, to see where that info gets to)

https://github.com/llvm/llvm-project/pull/121394