- This is how the real kernel works, and is more accurate and simpler.

Allows the compiler to warn about cases where the constructor is used
but then immediately discarded, which is a potential cause of
locking/unlocking bugs.

Profiling shows that this is a highly contested mutex, causing dimishing
results compared to a OS lock. std::mutex implementations can spin for a
while before falling back to an OS lock.

This avoids wasting precious CPU cycles in a no-op.

This commit: Implements CPU Interrupts, Replaces Cycle Timing for Host 
Timing, Reworks the Kernel's Scheduler, Introduce Idle State and 
Suspended State, Recreates the bootmanager, Initializes Multicore 
system.

Over the course of the changes to the kernel code, a few includes are no
longer necessary, particularly with the change over to std::shared_ptr
from Boost's intrusive_ptr.

* kernel: Replace usage of boost::intrusive_ptr with std::shared_ptr for kernel objects.

- See https://github.com/citra-emu/citra/pull/4710 for details.

Uncovered a bug within Thread's SetCoreAndAffinityMask() where an
unsigned variable (ideal_core) was being compared against "< 0", which
would always be a false condition.

We can also get rid of an unused function (GetNextProcessorId) which contained a sign
mismatch warning.

This is only compared against, so it can be made const.

Reduces the overall amount of code.

Adjusts the formatting of a few of the comments an ensures they get
recognized as proper Doxygen comments.

In case of redundant yields, the scheduler will now idle the core for 
it's timeslice, in order to avoid continuously yielding the same thing 
over and over.