core/debugger: fix a number of shutdown deadlocks

src/core/debugger/debugger_interface.h

@@ -66,6 +66,11 @@ public:
      */
     virtual void Stopped(Kernel::KThread* thread) = 0;
 
+    /**
+     * Called when emulation is shutting down.
+     */
+    virtual void ShuttingDown() = 0;
+
     /**
      * Called when new data is asynchronously received on the client socket.
      * A list of actions to perform is returned.