Kernel: Add formal Processor::verify_no_spinlocks_held() API

In a few places we check `!Processor::in_critical()` to validate
that the current processor doesn't hold any kernel spinlocks.

Instead lets provide it a first class name for readability.
I'll also be adding more of these, so I would rather add more
usages of a nice API instead of this implicit/assumed logic.

Kernel/Arch/aarch64/Processor.h

@@ -147,6 +147,11 @@ public:
         return current().m_in_critical;
     }
 
+    ALWAYS_INLINE static void verify_no_spinlocks_held()
+    {
+        VERIFY(!Processor::in_critical());
+    }
+
     // FIXME: Actually return the idle thread once aarch64 supports threading.
     ALWAYS_INLINE static Thread* idle_thread()
     {