[clang] [RISCV] Allow YAML file to control multilib selection (PR #98856)

[Sam Elliott via cfe-commits]

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

I'm happy with this as a good first step to adopting the multilib YAML.

Have you, using this patch, managed to reimplement the hardcoded multilib behaviour? This is not a blocking question, but it might give an indication of whether more work will be needed before this is widely usable. 

https://github.com/llvm/llvm-project/pull/98856