[compiler-rt] [compiler-rt][builtins] Switch libatomic locks to pthread_mutex_t. (PR #94374)

[Fangrui Song via llvm-commits]

MaskRay wrote:

It seems that we forgot PTHREAD_MUTEX_INITIALIZER. Perhaps we should avoid false sharing as well:
https://github.com/llvm/llvm-project/pull/94374

https://github.com/llvm/llvm-project/pull/94374