THE MASSIVE CABORA BASSA dam now being built in Portuguese Mozambique provokes some strong passions. For its many opponents across the world, it is a concrete symbol of white racialist determination to retain power in southern Africa.   South Africa has come north from her natural defence line to help build the dam, pay the lion’s share of its costs, and take most of the electricity it will produce, while Mozambique and Rhodesia will also gain much mutual benefit from the grandiose development scheme of which the dam is the vital core.