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/Forward.h

@@ -59,6 +59,7 @@ class Scheduler;
 class SharedBuffer;
 class Socket;
 template <typename BaseType> class SpinLock;
+class RecursiveSpinLock;
 template <typename BaseType, typename LockType> class ScopedSpinLock;
 class TCPSocket;
 class TTY;