Kernel: Do not return a null ref in Processor::current on aarch64

Clang rejects binding a reference to a null pointer at compile-time.
Let's just crash explicitly, instead of waiting for a null dereference
to mess things up.

Kernel/Arch/aarch64/Processor.h

@@ -58,7 +58,7 @@ public:
         return 0;
     }
 
-    ALWAYS_INLINE static Processor& current() { return *((Processor*)0); }
+    ALWAYS_INLINE static Processor& current() { VERIFY_NOT_REACHED(); }
 
     static void deferred_call_queue(Function<void()> /* callback */) { }