Assert that SystemDictionaryShared::find_builtin_class is not called twice for the same class

In the below code, we should never have two concurrent threads that each obtain the same non-NULL ik. Or else they would proceed to restore the same ik twice and will run into lots of issues.

There's already code in the SystemDictionary to prevent this from happening. We should add an appropriate assert to make this clear.

InstanceKlass* SystemDictionary::load_instance_class_impl(Symbol* class_name, Handle class_loader, TRAPS) {

    if (UseSharedSpaces)
    {
      PerfTraceTime vmtimer(ClassLoader::perf_shared_classload_time());
      InstanceKlass* ik = SystemDictionaryShared::find_builtin_class(class_name);
      if (ik != NULL && ik->is_shared_boot_class() && !ik->shared_loading_failed()) {
        SharedClassLoadingMark slm(THREAD, ik);
        k = load_shared_class(ik, class_loader, Handle(), NULL, pkg_entry, CHECK_NULL);
      }
    }