[clang] [compiler-rt] [llvm] [CMake][compiler-rt] Support for using compiler-rt atomic library (PR #106603)

[Chris Apple via cfe-commits]

cjappl wrote:

> but it'd be better if we handled atomic CAS spinlocks as well since we would like to avoid the pthread dependency if possible.

Agreed, I have something in the works for this but it is a little ways off. (catching these unbound loops is a bit tricky)

My recommendation to make it work when using the builtins would be just extracting this test to another test, then #ifdefing it out in the situation where we are not using pthread

https://github.com/llvm/llvm-project/pull/106603