This is done by adding a new zend_atomic_bool type. The type
definition is only available for compiler alignment and size info; it
should be treated as opaque and only the zend_atomic_bool_* family of
functions should be used.
Note that directly using atomic_bool is complicated. All C++ compilers
stdlibs that I checked typedef atomic_bool to std::atomic<bool>, which
can't be used in an extern "C" section, and there's at least one usage
of this in core, and probably more outside of it.
So, instead use platform specific functions, preferring compiler
intrinsics.
