atomic_init() and friends failing on clang

Clang does not like how we're using atomic_init() and friends in src/lib/thread/threads.h. It looks like these changes were introduced by nickm. I'm not finding a lot of great documentation on these functions, though.

https://jenkins.torproject.org/job/tor-ci-linux-master-clang/3164/ARCHITECTURE=i386,SUITE=sid/consoleFulll https://jenkins.torproject.org/view/Failed+Unstable/job/tor-ci-linux-0.3.3-clang/lastFailedBuild/ARCHITECTURE=i386,SUITE=sid/console

0.3.4 also, but lost the url in the jenkins maze.

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