What do people in prison do if they're claustrophobic?

You are approaching this problem from the wrong direction. Claustrophobic people should avoid committing crimes, so that they do not find themselves locked into small cells where their claustrophobia will become a problem. It is not up to the penal system to accommodate claustrophobic people. Claustrophobic people get locked up just like anyone else, if they have been found guilty of a crime. If this drives them crazy, well, prison drives lots of people crazy.