The US is airlifting emergency arms shipments to Thailand. Four cases of ground-to-air missiles arrived this weekend in the face of cross-border raids by Vietnamese troops pursuing Kampuchean rebels. Two giant C- 5 cargo planes were to fly in Monday with eight 155mm howitzers, modified to extend their range to match the long-range 130mm Soviet-made artillery of the Vietnamese. Not surprisingly, the US shipments were condemned by Vietnam, which said the arms would aggravate the border situation. It also accused the US of coordinating with China to bring about confrontation in the region.