Hello,
apparently qemu/kvm which is used in my case does not expose that flag by default. Or the debian guest does not pick it up for some reason. A quick google does not come up with something directly useful why this flag might be missing. And the debian guest apparently does not care 
EDIT: Or the CPU which is emulated (Conroe/Merom Class Core 2) is really to old, if I understand correctly @flicaflow uses virtualbox
EDIT2: Well, apparently the penryn 45 nm die shrink is the first CPU with cx16, the merom 65 nm (and presumably the 65 nm conroe) does not have it. Well, bad luck