What are the 'acidity controllers' used in canned whole tomatoes?
It is normally just a mild organic acid like citric acid, they tend to act as buffers. But the reason to use a word like "acidity controller" is exactly to be able to use something else (say malic acid) so they don't have to reprint the label if they change the recipe. 

The reasons for acidity controllers is both safety (not enough acidity will make the can unsafe) and taste - too much or too little acidity will result in a taste difference between charges, because each batch of tomatoes comes with different acidity. 

Basic buffers like baking soda cannot be used, because this would make the contents practically uncannable, and very unpleasant to eat to boot.