KUALA LUMPUR, Malaysia — A piece of a wing found washed up on Reunion Island last week is from Malaysia Airlines Flight 370 that vanished last year, Malaysia’s prime minister announced early Thursday, saying he hoped the news would end the “unspeakable” uncertainty of the passengers’ families. The disappearance of the Boeing 777 jetliner 515 days ago while on a flight from Kuala Lumpur to Beijing on March 8, 2014, has been one of the biggest mysteries in aviation history.