Make legality check in GPU->SPIR-V lowering of FuncOp kernel specific.

Existing check that sets FuncOp to be dynamically legal was just
checking that the types of the argument are SPIR-V compatible. Since
the current conversion from GPU to SPIR-V does not handle lowering
non-kernel functions, change the legality check to verify that the
FuncOp has the gpu.kernel attribute and has void(void) return type.

PiperOrigin-RevId: 280032782