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

[Sam Elliott via cfe-commits]

lenary wrote:

@ArcaneNibble have you seen this RFC from Arm https://discourse.llvm.org/t/rfc-multilib-custom-flags/81058?u=lenary which I think would be useful for RISC-V too?

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