[compiler-rt][builtins] Switch libatomic locks to pthread_mutex_t (#94374) When an uninstrumented libatomic is used with a TSan instrumented memcpy, TSan may report a data race in circumstances where writes are arguably safe. This occurs because __atomic_compare_exchange won't be instrumented in an uninstrumented libatomic, so TSan doesn't know that the subsequent memcpy is race-free. On the other hand, pthread_mutex_(un)lock will be intercepted by TSan, meaning an uninstrumented libatomic will not report this false-positive. pthread_mutexes also may try a number of different strategies to acquire the lock, which may bound the amount of time a thread has to wait for a lock during contention. While pthread_mutex_lock has a larger overhead (due to the function call and some dispatching), a dispatch to libatomic already predicates a lack of performance guarantees.
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