The MinGW-w64 32-bit .def files shouldn't be used with MinGW-w64 64-bit.
Rename .def files from <library>_mingw.def to <library>_mingw32.def and
only use them if QT_ARCH is i386.

Change-Id: I5dc2a87f7d2ed59bf954faf2983d0787ed9e00e9
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Patrick von Reth <vonreth@kde.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>