tsan: Implement pthread_exit() interceptor for Thread sanitizer

This change is preparation for fiber support.

Author: yuri (Yuri Per)
Reviewed in: https://reviews.llvm.org/D57876
Context: https://reviews.llvm.org/D54889

llvm-svn: 353385

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@353627 91177308-0d34-0410-b5e6-96231b3b80d8