[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

3rdparty/CMakeLists.txt

@@ -357,6 +357,9 @@ add_subdirectory(opencv EXCLUDE_FROM_ALL)
 # FUSION
 add_subdirectory(fusion EXCLUDE_FROM_ALL)
 
+# FERAL INTERACTIVE
+add_subdirectory(feralinteractive EXCLUDE_FROM_ALL)
+
 # add nice ALIAS targets for ease of use
 if(USE_SYSTEM_LIBUSB)
 	add_library(3rdparty::libusb ALIAS usb-1.0-shared)
@@ -389,3 +392,4 @@ add_library(3rdparty::miniupnpc ALIAS libminiupnpc-static)
 add_library(3rdparty::rtmidi ALIAS rtmidi)
 add_library(3rdparty::opencv ALIAS ${OPENCV_TARGET})
 add_library(3rdparty::fusion ALIAS Fusion)
+add_library(3rdparty::feralinteractive ALIAS 3rdparty_feralinteractive)