Fix bug in option propagation.
Fix bug in option propagation.

Used without {} SUPPORTED_ARCHITECTURES's name is used as a value, resulting in
not propagating any of the option definitions. Also added diagnostic to be
explicit about supported architectures.

AmesianX/capstone,fvrmatteo/capstone,bSr43/capstone,bSr43/capstone,xia0pin9/capstone,xia0pin9/capstone,bSr43/capstone,bSr43/capstone,bSr43/capstone,xia0pin9/capstone,AmesianX/capstone,AmesianX/capstone,bSr43/capstone,fvrmatteo/capstone,AmesianX/capstone,fvrmatteo/capstone,fvrmatteo/capstone,xia0pin9/capstone,fvrmatteo/capstone,fvrmatteo/capstone,AmesianX/capstone,fvrmatteo/capstone,AmesianX/capstone,xia0pin9/capstone,xia0pin9/capstone,bSr43/capstone,xia0pin9/capstone,AmesianX/capstone