[msan] Get stack limits with pthread_create interceptor.

Before we did it lazily on the first stack unwind in the thread.
It resulted in deadlock when the unwind was caused by memory allocation
inside pthread_getattr_np:
  pthread_getattr_np   <<< not reentable
  GetThreadStackTopAndBottom
  __interceptor_realloc
  pthread_getattr_np
  

llvm-svn: 197026