[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
  


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@197026 91177308-0d34-0410-b5e6-96231b3b80d8