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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This also broke builds for Windows: https://github.com/mstorsjo/llvm-mingw/actions/runs/9376329007/job/25816083980

> Can you specialize this change to fall back to `_Atomic` when `COMPILER_RT_HAS_LIBPTHREAD` is off?

This sounds like a good idea, but I'd also like to make it dependent on platform. E.g. on Windows, libpthread may exist - there are third party libraries that provide the pthreads API, but we don't want to use it here even if it happens to exist while building compiler-rt.

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