bpo-30747: Attempt to fix atomic load/store (#2383)

_Py_atomic_* are currently not implemented as atomic operations
when building with MSVC. This patch attempts to implement parts
of the functionality required.

Misc/NEWS.d/next/Core and Builtins/2017-08-08-12-00-29.bpo-30747.g2kZRT.rst

+Add a non-dummy implementation of _Py_atomic_store and _Py_atomic_load on
+MSVC.