Prevent Thread Exited/Joined events race

Summary:
Add atomic verification to ensure that Thread is Joined after marking it
Finished.

It is required for NetBSD in order to prevent Thread Exited/Joined race,
that may occur when native system libpthread(3) cannot be reliably traced
in a way to guarantee that the mentioned events happen one after another.

This change fixes at least TSan and LSan on NetBSD.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dvyukov, vitalybuka

Reviewed By: dvyukov

Subscribers: llvm-commits, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40294

llvm-svn: 319004