Hi,here the issue:
We (crystax.net) use custom builds of cpython,
which for windows are compiled by MinGW with pyconfig.h taken from PC/pyconfig.h
And for 32-bit Windows everything works well, but for 64-bit Windows - doesn't.

The root cause of this issue is actauly very simple:

Python code for windows is very sensitive on properly defined macro MS_WIN32/MS_WIN64
And while MS_WIN32 is predefined unconditionally in PC/pyconfig.h,
the MS_WIN64 is defined only in conjunction with _MSC_VER, like

#ifdef _MSC_VER
...
#ifdef _WIN64
#define MS_WIN64
#endif
...
#endif /* _MSC_VER */

So suggested patch (for 3.5.2 and 2.7.12) just appropriately define MS_WIN64 for MinGW