Use C++ atomics for the implementation of gutil/atomicops.h

Chromium switched the implementation of atomicops.h and family to use the C++11 atomics library in this change:

https://github.com/chromium/chromium/commit/57a4e4a50c673c25e9cdaab53e32f6e53aa0b574

It allowed for a large simplification later on where most of the platform-specific logic and assembly could be removed. This seems easier to maintain than the current code.