Kernel: Optimize scheduler idling

Now after each interrupt we will ask the scheduler to reschedule
if the current thread is the idle thread. This allows semaphore
unblocking to be practically instant when there is only one thread
executing.

Now disk reading is back to ~3 MB/s for single threaded process

kernel/include/kernel/Scheduler.h

@@ -16,6 +16,7 @@ namespace Kernel
 
 		void start();
 		void reschedule();
+		void reschedule_if_idling();
 
 		BAN::ErrorOr<void> add_thread(Thread*);