Kernel: Introduce ProcessorSpecific<T> for per-CPU data structures

To add a new per-CPU data structure, add an ID for it to the
ProcessorSpecificDataID enum.

Then call ProcessorSpecific<T>::initialize() when you are ready to
construct the per-CPU data structure on the current CPU. It can then
be accessed via ProcessorSpecific<T>::get().

This patch replaces the existing hard-coded mechanisms for Scheduler
and MemoryManager per-CPU data structure.

Kernel/Forward.h

@@ -55,7 +55,7 @@ class RangeAllocator;
 class RecursiveSpinLock;
 class Region;
 class Scheduler;
-class SchedulerPerProcessorData;
+class SchedulerData;
 class SharedInodeVMObject;
 class Socket;
 class Space;