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/Core.cpp

@@ -327,6 +327,8 @@ void EmuThread()
 
 	Common::SetCurrentThreadName("Emuthread - Starting");
 
+	if (SConfig::GetInstance().m_OCEnable)
+		DisplayMessage("WARNING: running at non-native CPU clock! Game may not be stable.", 8000);
 	DisplayMessage(cpu_info.brand_string, 8000);
 	DisplayMessage(cpu_info.Summarize(), 8000);
 	DisplayMessage(core_parameter.m_strFilename, 3000);