atexit callbacks need to be threadsafe

[behdad]

Apparently POSIX doesn't specify whether atexit callbacks are called before or after threads are terminated during process shutdown. Following suggests that in fact, they are called while threads still running!

https://stackoverflow.com/questions/39655868/what-does-the-posix-standard-say-about-thread-stacks-in-atexit-handlers-what

We should make our atexit callbacks threadsafe, simply by using cmpexch to clean things up.