[tsan] Allow memchr interceptor to be used before initialization on OS X

On OS X, `memchr` is called on a newly created thread even before `__tsan_thread_start_func` is invoked, which means that the ThreadState object for that thread will not yet be initialized. Let's add `COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED` into the interceptor to simply call `internal_memchr` in these cases.

Differential Revision: http://reviews.llvm.org/D14283

llvm-svn: 251935