[clang] [llvm] [Driver][RISC-V] Support YAML multilib config with selectRISCVMultilib fallback (PR #184973)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

> > a user requesting rv32imc won't match an rv32im variant even though it is a safe fallback
> 
> I'm not sure this assertion is true, given we are working on a toolchain that uses the multilib yaml and does manage to do this.
> 
> Let me get back to you tomorrow with how we made this work.

My description might not correct, it actually can be done by `Mappings`/`Match` with regular expression, but that would be loooong expression I think, anyway, wait your feedback :)

https://github.com/llvm/llvm-project/pull/184973