- removed STACK_ARGS.

The only reason this even existed was that ZDoom's original VC projects used __fastcall. The CMake generated project do not, they stick to __cdecl.
Since no performance gain can be seen by using __fastcall the best course of action is to just remove all traces of it from the source and forget that it ever existed.

src/posix/sdl/i_system.cpp

@@ -179,7 +179,7 @@ bool gameisdead;
 void Mac_I_FatalError(const char* errortext);
 #endif
 
-void STACK_ARGS I_FatalError (const char *error, ...)
+void I_FatalError (const char *error, ...)
 {
     static bool alreadyThrown = false;
     gameisdead = true;
@@ -216,7 +216,7 @@ void STACK_ARGS I_FatalError (const char *error, ...)
     }
 }
 
-void STACK_ARGS I_Error (const char *error, ...)
+void I_Error (const char *error, ...)
 {
     va_list argptr;
     char errortext[MAX_ERRORTEXT];