winrt: Fix potential crash in QCoreApplication

GetModuleFileName exists for Windows 10 and upwards, hence use the
generic version from the win32 mkspec. This allows to create a
QCoreApplication object with nullptr argv, as the application filename
is identified via the binary itself and not via arguments. A couple of
auto-tests use this method to create multiple application objects during
runtime.

Unfortunately we cannot apply this for msvc2013, even though MSDN states
the GetModuleFileName exists, it fails to compile for Windows Phone 8.1.

Change-Id: I2b8b988107487ef3785462f8ca40b0a6e0623e32
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>