Avoids directly relying on the global system instance and instead makes
an arbitrary system instance an explicit dependency on construction.

This also allows removing dependencies on some global accessor functions
as well.

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.

Keeps the CPU-specific behavior from being spread throughout the main
System class. This will also act as the home to contain member functions
that perform operations on all cores. The reason for this being that the
following pattern is sort of prevalent throughout sections of the
codebase:

If clearing the instruction cache for all 4 cores is necessary:

Core::System::GetInstance().ArmInterface(0).ClearInstructionCache();
Core::System::GetInstance().ArmInterface(1).ClearInstructionCache();
Core::System::GetInstance().ArmInterface(2).ClearInstructionCache();
Core::System::GetInstance().ArmInterface(3).ClearInstructionCache();

This is kind of... well, silly to copy around whenever it's needed.
especially when it can be reduced down to a single line.

This change also puts the basics in place to begin "ungrafting" all of the
forwarding member functions from the System class that are used to
access CPU state or invoke CPU-specific behavior. As such, this change
itself makes no changes to the direct external interface of System. This
will be covered by another changeset.