core/core_timing: Make callback parameters consistent

In some cases, our callbacks were using s64 as a parameter, and in other
cases, they were using an int, which is inconsistent.

To make all callbacks consistent, we can just use an s64 as the type for
late cycles, given it gets rid of the need to cast internally.

While we're at it, also resolve some signed/unsigned conversions that
were occurring related to the callback registration.