I'm not sure how these two are related. Why would you mix modulo operations and fixed size? Modulo on an arbitrary bigint is perfectly valid and useful.
Modulo arithmetic refers to the property of fixed-size integers where they will predictably wraparound in expected ways if you overflow or underflow them. This wraparound is "free", so it used to be commonly used for cyclic high performance code. An arbitrary bigint is computationally more expensive, and it will never experience this effect, so it's a double whammy of irrelevance for bigints. You can manually do a modulo operation on a bigint, but that just makes them even more computationally expensive than a fixed size int, requires more code, and would be considered an unnecessary inconvenience by most people who care about modulo arithmetic.
If you're relying on modulo arithmetic, you should be using a fixed size integer anyways, not one that varies based on the architecture. Go has fixed size integer types available, and modulo arithmetic should be using those, not "int".