[llvm] [Exegesis] CPU selection, when native arch and target mismatch (PR #131014)

[via llvm-commits]

dyung wrote:

@AnastasiyaChernikova the test you added is failing on several bots. I believe the problem is you are assuming the riscv target is being built, and the test fails on bots where it is not being built (at least for my bots). Can you take a look and fix or revert if you need time to investigate?

https://github.com/llvm/llvm-project/pull/131014