[tsan] Alternative ThreadState storage for OS X

This implements a "poor man's TLV" to be used for TSan's ThreadState on OS X. Based on the fact that `pthread_self()` is always available and reliable and returns a valid pointer to memory, we'll use the shadow memory of this pointer as a thread-local storage. No user code should ever read/write to this internal libpthread structure, so it's safe to use it for this purpose. We lazily allocate the ThreadState object and store the pointer here.

Differential Revision: http://reviews.llvm.org/D14288

llvm-svn: 252159