core/debugger: fix a number of shutdown deadlocks

src/core/core.h

@@ -160,6 +160,9 @@ public:
     /// Shutdown the emulated system.
     void Shutdown();
 
+    /// Forcibly detach the debugger if it is running.
+    void DetachDebugger();
+
     std::unique_lock<std::mutex> StallCPU();
     void UnstallCPU();