Kernel: MMU::get() is now MMU::kernel

MMU is can now be locked with RecursiveSpinLock.

Scheduler now has get_current_tid() that works before the Scheduler
is initialized. This allows RecursiveSpinLock usage early on.

kernel/include/kernel/Scheduler.h

@@ -26,6 +26,7 @@ namespace Kernel
 		void unblock_threads(Semaphore*);
 
 		Thread& current_thread();
+		static pid_t current_tid();
 
 	private:
 		Scheduler() = default;