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

[via llvm-commits]

pirama-arumuga-nainar wrote:

This PR broke the armv8-a builtins build in Android toolchain - which can lower `_Atomic` but the baremetal environment doesn't have `<pthread.h>`.  Can you specialize this change to fall back to `_Atomic` when `COMPILER_RT_HAS_LIBPTHREAD` is off?

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