Option 1: you can
disown the process so it won't die when its parent (the terminal session) gets killed, by changing your command slightly:
LD_PRELOAD='/usr/$LIB/libstdc++.so.6' steam & disown
Option 2: you can edit the .desktop file for Steam to make its launcher use the command you normally use, so you can start it by clicking the icon instead of launching Steam from a terminal.
Open a terminal and type
Make a backup copy of the
sudo cp steam.desktop steam-bak
(you can restore the backup if needed by doing
mv steam-bak steam.desktop) edit the original file:
sudo nano steam.desktop
find the first line that starts
Exec= (you can use ctrl+w to search in nano) and edit the line so it says:
Exec=bash -c "LD_PRELOAD='/usr/$LIB/libstdc++.so.6' steam"
Save the file and exit (ctrl+o then enter then ctrl+x)