Fix GCRefMap for generic method calls via Unboxing stub (#1304)

Fix GCRefMap for generic method calls via Unboxing stub (#1304)

Crossgen2 was generating GCRefMap for calls to generic methods via
unboxing stubs incorrectly. With unboxing stub, the caller doesn't pass
in an extra generic parameter since the reference to the boxed value
type passed to the stub serves that purpose. But the GetCallRefMap method
was generating GCRefMap entry for the extra generic argument, which was
leading to a crash when GC was scanning the arguments.