Kernel: Use separate Handle tables for CoreTiming userdata

This is to support the removal of GetHandle soon

src/core/hle/kernel/timer.h

@@ -50,6 +50,9 @@ public:
 
 private:
     Timer() = default;
+
+    /// Handle used as userdata to reference this object when inserting into the CoreTiming queue.
+    Handle callback_handle;
 };
 
 /// Initializes the required variables for timers