steamrt-dev: Add g++-5-multilib, gcc-5-multilib
Some games that previously targeted SteamRT 1 'scout' specifically want to use gcc 5.
For https://phabricator.collabora.com/T14618
Requires https://gitlab.collabora.com/steam/gcc-5/merge_requests/4
/cc @timothee.besset-ext @zumbi @kanashiro
Edited by Lucas Kanashiro