Japan has given its official backing to the Panamanian government controlled by military strongman Gen. Manuel Antonio Noriega and the United States has quietly accepted the decision, Japanese officials said Friday. Japan's support could give Noriega a needed financial boost in his battle to survive pressure from President Reagan to leave office. In Washington, Deputy White House press secretary Roman Popadiuk said the U.S. has not been officially notified of Japanese recognition of the Panamanian government.