Support overclocking and underclocking the Gamecube CPU

Won't work with all games, but provides a nice way to spend extra CPU to make
a variable framerate game faster (e.g. Spyro or The Last Story), or to make
a game use less CPU at the cost of a lower framerate (e.g. Rogue Leader).

Source/Core/Core/ConfigManager.h

@@ -49,6 +49,8 @@ struct SConfig : NonCopyable
 	int m_InterfaceLanguage;
 	// framelimit choose
 	unsigned int m_Framelimit;
+	bool m_OCEnable;
+	float m_OCFactor;
 	// other interface settings
 	bool m_InterfaceToolbar;
 	bool m_InterfaceStatusbar;