Waves of U.S. helicopters airlifted Americans from wartorn Cambodia to an aircraft carrier in the Gulf of Siam today before the expected fall of Phnom Penh to Communist-led insurgents. Three dozen Air Force and Marine helicopters flew 289 persons, a third of them Americans, to the carrier Okinawa circling off the Cambodian coast. The choppers, guarded by a 346-man Marine battalion and a squadron of 20 warplanes, also flew out 159 Cambodians, including Acting President Sau Kham Khoy. The