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

[Simon Cook via cfe-commits]

simonpcook wrote:

I think the fake flag hack may have a limitation in that since the `--XXX-fake-flag-...` appears in `--print-multi-lib` it may get imported into some build system and used as a flag for building particular libraries. For example with the above, newlib will not build since `error: unknown argument: '--XXX-fake-flag-yaml-has-m'`.

Of course, I can build newlib with a different multilib.yaml file that I later distribute the tools with - more an FYI for anyone who hits that one in the future.

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