TOKYO (Reuters) - U.S. hedge fund ValueAct Capital has become a major shareholder in Japanese medical equipment and camera maker Olympus Corp (7733.T) with a 5.04 percent stake that is worth around $612 million at current shares prices. The stake hike comes at a time when activist investment is gaining momentum in Japan, with Prime Minister Shinzo Abe advocating shareholders raise issues and have constructive dialogue with firms to strengthen corporate governance.