[System] Fix restart-loops upon restart-then-quit (#18723)

I observed under macOS (although the bug seems platform agnostic to me)
that when restarting a game, then subsequently quitting it, I'd get
stuck in a 'restart loop' where the game would restart upon completely
shutting down, and this would repeat every time I tried to close the
game until I gave up & force quit RPCS3. Seems like for whatever reason,
the use of std::move didn't actually guarantee that after_kill_callback
got nuked if at all, so all I've done is take a hammer to the fucker and
manually set it to nullptr after being called.

Co-authored-by: Elad <18193363+elad335@users.noreply.github.com>

rpcs3/Emu/System.cpp

@@ -4068,8 +4068,7 @@ void Emulator::Kill(bool allow_autoexit, bool savestate, savestate_stage* save_s
 			if (after_kill_callback)
 			{
 				// Make after_kill_callback empty before call
-				const auto callback = std::move(after_kill_callback);
+				std::exchange(ensure(after_kill_callback), nullptr)();
-				callback();
 			}
 		});
 	}));
@@ -4125,7 +4124,7 @@ game_boot_result Emulator::Restart(bool graceful, bool reset_path)
 	else
 	{
 		// Execute and empty the callback
-		::as_rvalue(std::move(Emu.after_kill_callback))();
+		std::exchange(ensure(Emu.after_kill_callback), nullptr)();
 	}
 
 	return game_boot_result::no_errors;