[class] Short circuit collect_implemented_interfaces_aux on gparams

[class] Short circuit collect_implemented_interfaces_aux on gparams

and also setup_interface_offsets.

A gparam is never a real interface so we can just short circuit
if we see a gparam during interface collection.  (A gparam may be instantiated
with an interface --- but ginst interface offsets are processed anew when the
ginst is initialized in mono_class_init, not somehow extracted from the gtd and
the MonoGenericContext).