[sanitizer] Use pthread_threadid_np as thread ID on OS X

Let's use pthread_threadid_np which returns a more reasonable ID than pthread_self (which is actually a stack pointer). The numbers from pthread_threadid_np are already used in other tools, e.g. in LLDB, and often appear in logs, so it's much more useful than pthread_self.

Differential Revision: http://reviews.llvm.org/D18951

llvm-svn: 266991