SDK: Add C and C++ multilib (-m32) support for all gcc versions
We use a new example project with no dependencies to test the C path, because both GLib and SDL are unavailable for -m32 in our biarch containers due to multiarch co-installability issues that were only fixed in Debian 10 or later.
(T21490)
Test-build: https://jenkins.internal.steamos.cloud/job/playground/job/scout/job/build/266/
Edited by Simon McVittie