[Linux] Implement Feral Interactive's Gamemode for Potential Performance Increases (#17325)

Currently, this is a draft PR to implement Feral Interactive's Gamemode
into RPCS3, which can improve game performance on certain Linux systems.

At the moment, I am running into various compiler warnings when trying
to include "gamemode_client.h" due to the file not using strict typings
and old C-style casts. I know I can disable these flags during RPCS3's
compilation but I wanted to check with the maintainers before going
forward and if this feature should be implemented.

It should be noted that RPCS3 only fails to compile and run if I include
Feral's header file, but everything else compiles if I comment out the
include.

Targets Issue #11299

rpcs3/Emu/System.cpp

@@ -2477,6 +2477,11 @@ void Emulator::Run(bool start_playtime)
 	{
 		Emu.GetCallbacks().enable_display_sleep(false);
 	}
+
+	if (g_cfg.misc.enable_gamemode)
+	{
+		Emu.GetCallbacks().enable_gamemode(true);
+	}
 }
 
 void Emulator::RunPPU()
@@ -3237,6 +3242,12 @@ void Emulator::Kill(bool allow_autoexit, bool savestate, savestate_stage* save_s
 
 	sys_log.notice("Stopping emulator...");
 
+	// Calling Gamemode Exit on Stop
+	if (g_cfg.misc.enable_gamemode)
+	{
+		Emu.GetCallbacks().enable_gamemode(false);
+	}
+
 	const bool continuous_savestate_mode = savestate && !g_cfg.savestate.suspend_emu;
 
 	// Show visual feedback to the user in case that stopping takes a while.