Kernel: Implement software context switching and Processor structure

Moving certain globals into a new Processor structure for
each CPU allows us to eventually run an instance of the
scheduler on each CPU.

Kernel/Interrupts/APIC.h

@@ -87,6 +87,7 @@ private:
 
     OwnPtr<Region> m_apic_base;
     NonnullOwnPtrVector<Region> m_apic_ap_stacks;
+    Vector<Processor> m_ap_processor_info;
     AK::Atomic<u32> m_apic_ap_count{0};
     
     static PhysicalAddress get_base();