8000659: NPG: ClassCastExceptions are unexpectedly thrown when testing nashorn

Summary: Treat the oops in invoke_method_table() as strong roots when ClassUnloading is enabled.
Reviewed-by: kamg, coleenp

src/share/vm/classfile/systemDictionary.cpp

@@ -1749,6 +1749,9 @@ void SystemDictionary::always_strong_oops_do(OopClosure* blk) {
   blk->do_oop(&_system_loader_lock_obj);
 
   dictionary()->always_strong_oops_do(blk);
+
+  // Visit extra methods
+  invoke_method_table()->oops_do(blk);
 }
 
 void SystemDictionary::always_strong_classes_do(KlassClosure* closure) {