spu: Speed hacks - Do not starve PPU threads

optionally hint to the OS scheduler to give less attention to SPUs

ui: Add speed 'hacks' as configurable options

Utilities/Thread.cpp

@@ -1750,6 +1750,7 @@ void named_thread::start_thread(const std::shared_ptr<void>& _this)
 		try
 		{
 			LOG_TRACE(GENERAL, "Thread started");
+			on_spawn();
 			on_task();
 			LOG_TRACE(GENERAL, "Thread ended");
 		}

Utilities/Thread.h

@@ -258,6 +258,9 @@ protected:
 	// Thread finalization (called after on_task)
 	virtual void on_exit() {}
 
+	// Called once upon thread spawn within the thread's own context
+	virtual void on_spawn() {}
+
 public:
 	// ID initialization
 	virtual void on_init(const std::shared_ptr<void>& _this)