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.
Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.
Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.
Places all of the timing-related functionality under the existing Core
namespace to keep things consistent, rather than having the timing
utilities sitting in its own completely separate namespace.
LOG_GENERIC usages will be amended in a follow-up to keep API changes separate from
interface changes, as it will require removing a parameter from the relevant function
in the VMManager class.
