[llvm] [llvm-exegesis] Fix LBR hang on hybrid x86 CPUs (PR #216917)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Where exactly are you running into this? Hangs with just check-llvm?
> 
> If that's the case, then I think I'd probably just prefer a band aid of timing out during test discovery (e.g., timeout of 5s). To fix this properly I think we would need much more principled handling of CPU cores that we don't currently have. I'm also not aware of anyone using the LBR functionality.

Band-aid: https://github.com/llvm/llvm-project/pull/218102

https://github.com/llvm/llvm-project/pull/216917