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

[Sam Elliott via llvm-commits]

lenary wrote:

One thing while I'm thinking about this: There are likely other clang driver flags that we want to propagate to multilib selection for risc-v. One we found recently was the shadow stack stuff (#182350) - but I'm sure there are more, and we should add them when we find them.

For instance Arm/AArch64 add their alignment-related flags, which we probably should too.

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