[compiler-rt] [compiler-rt] Moved cmake libatomic check to top level config-ix (PR #99437)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This looks good in the absence of further response.

I am not very experienced with CMake, so I waited a bit in case others have opinions... I think this should be safe.

https://github.com/llvm/llvm-project/pull/99437