RIYADH (Reuters) - The United States will work with Gulf Arab nations to strengthen their shared defenses against threats including Iran, U.S. Secretary of State Hillary Clinton said on Saturday. Speculation about Tehran's nuclear ambitions is a source of anxiety among Sunni-led Gulf Arab states, for whom Shi'ite Iran has long been a regional rival. "The commitment of the United States to the people and the nations of the Gulf is rock-solid and unwavering. ...