Kernel: Create CriticalScope and fix kmalloc

This disables interrupts for the current scope and restores them
after the scope. This is used in kmalloc, since scheduler might
call into kmalloc/kfree, but deadlock if some thread is currently
trying to allocate. This allows us to use kmalloc in Scheduler.

kernel/include/kernel/InterruptController.h

@@ -18,6 +18,4 @@ public:
 	static InterruptController& get();
 };
 
-uintptr_t disable_interrupts_and_get_flags();
-void restore_flags(uintptr_t);
 bool interrupts_enabled();