Support option(host_build) projects with cross-compiled Qt

QtCreator queries for the QMAKE_XSPEC from qmake and then uses that in qmake calls as -spec. This creates problem when using cross-compiled Qt and any project file that uses option(host_build), since it then tries to use QMAKE_XSPEC for the host parts as well.

One solution is to add support for -xspec in QtCreator and also in the sdktool.
Another option would be to not to use -spec when no mkspec is given for the Kit