tsan: catch more races on file descriptors



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@192452 91177308-0d34-0410-b5e6-96231b3b80d8
[sanitizer] Define kPthreadDestructorIterations on POSIX systems.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@192453 91177308-0d34-0410-b5e6-96231b3b80d8