I'm pretty sure that not all our platforms support std::atomic.  There's Common/Atomics.h though:

https://github.com/hrydgard/ppsspp/blob/master/Common/Atomic_GCC.h

Or, like I did here, you can use them only on platforms they're safe on (I just did Android):
https://github.com/hrydgard/ppsspp/blob/master/GPU/GPUCommon.h

Visual Studio 2010 doesn't actually support std::atomic, and that's our primary Windows target atm.
http://stackoverflow.com/questions/2936481/visual-c-2010-atomic-types-support

-[Unknown]