Fixed a race condition between the Watcher thread and the main thread during program exit.

[cgyrling]

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A race condition exist between the Watcher thread and main(). A case was found where the Watcher thread does not get execution time before the main function returns and calls atexit(). At that point the Watcher thread started runing tls_init() code while the main thread was shutting down. This resulted in rare crashes and deadlocks.

This fixes the following issue: (https://github.com/google/googletest/issues/4493)