Update mkspec format for winrt and winphone mkspecs

Change-Id: Ic5d94fa0d7819dc19e89fe157efc85f4e26de8aa
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Use common base mkspec for winrt and winphone specs

Similar to the Q_OS defines the winphone qmake scope
is set additionally to winrt. That is due to the fact
that they share similar system API but winphone offers
more than winrt alone (winsock 2 for example).

Change-Id: Icccc20803b129617ea6d3a092162293caa2667de
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>