First, I think this is going to depend on the university to some degree. If it's a university that's going to teach a lot of theory, the programming language of the courses is borderline unimportant as you're going to be learning more abstract concepts like data structures and algorithms (this will look familiar to you as a student of math). If, on the other hand, you're going to more of a trade school, I would suggest doing some tutorials on the language(s) in question and perhaps buy some books to work through.
Anecdotally, I did my undergrad work at Carnegie Mellon University, which is considered to be a rigorous institution, and I did not have any C/C++ experience when I went (that was the language of instruction at the time). This was not a significant barrier in the slightest. There was a track for students with C/C++ and another for students who had not done this before. I think you'll find the same to be the case at most institutions. If you have experience, great, they'll speed you along and cater to you. If not, they'll teach you.
I also would offer a piece of advice. Don't sweat a programming language. At a good school, you're going to learn mathematical and logical concepts, which you'll eventually narrow into focusing on specific programming concepts. Don't get me wrong -- you'll write code. But, you're not going to be learning how to crank out production software at a university like this -- you're going to learn to think like a Computer Scientist. These are similar, but not at all the same. That is to say, applied knowledge (specific languages) is not important in this context.