[tsan] Fix deadlock_detector_stress_test.cc testcase for OS X

On OS X, we don't have pthread spinlocks, let's just use a regular mutex instead. Secondly, pthread_rwlock_t is much larger (200 bytes), so `char padding_[64 - sizeof(pthread_rwlock_t)]` actually underflows.

Differential Revision: http://reviews.llvm.org/D14862

llvm-svn: 253659