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

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ArcaneNibble wrote:

> Could you add a testcase for that?

Added a test case for checking the resulting flags. There's already test cases covering handling the YAML file itself. I haven't added a test for the fallback logic specifically, as that seemed a bit fiddly and there's desire to remove it eventually.

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