Provide multiarch SDK to enable cross compilation

This is a first step towards allowing cross-compilation for flatpak (including using flatpak-builder).

The second part will be cross compiler SDK extensions. Those will be built in a separate projectbranch due to limitation in BuildStream dealing with artifacts built from a different architecture (uname cache key issue).

Please review along with !1250 (merged).

A branch of Steam flatpak app is available that uses this cross compiler to build shared-library-guard: https://github.com/valentindavid/com.valvesoftware.Steam/commit/08805f8227140d1abd74f20a2c21912fe554682a