[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/feralinteractive/CMakeLists.txt

@@ -0,0 +1,9 @@
+# Feral Interactive
+
+add_library(3rdparty_feralinteractive INTERFACE)
+
+if (CMAKE_SYSTEM MATCHES "Linux")
+	target_include_directories(3rdparty_feralinteractive INTERFACE feralinteractive/lib)
+	target_compile_definitions(3rdparty_feralinteractive INTERFACE -DGAMEMODE_AVAILABLE)
+	target_link_libraries(3rdparty_feralinteractive INTERFACE feralinteractive)
+endif()