SEOUL, South Korea — North Korean leader Kim Jong Un said Monday that the United States should be aware that his country’s nuclear forces are now a reality, not a threat. But he also struck a conciliatory tone in his New Year’s address, wishing success for the Winter Olympics set to begin in South Korea in February and suggesting the North may send a delegation to participate. Kim, wearing a Western-style gray suit and tie, said in his customary annual address that his country had achieved the historic feat of “completing” its nuclear forces and added that he has a nuclear button on his desk. “The U.S.