Qt: different options in .pro file based on target architecture in WinRT

Based on the target architecture different libraries (and other stuff) should be set. Therefore the .pro file includes this code:

contains(QMAKE_TARGET.arch, arm) {
    LIBS += -L$$PWD/arm
}
else {
    contains(QMAKE_TARGET.arch, x86_64) {
        LIBS += -L$$PWD/x64
    }
    else {
        LIBS += -L$$PWD/x86
    }
}

Now the problem is that contains(QMAKE_TARGET.arch, arm) is never true. Is there a different way to get the ARM architecture for WinRT builds?

Regards,