Cross-compile: add Debian Ports platforms

[Splendide-Imaginarius]

Adding targets supported by Debian Ports (e.g. MIPS and POWER BE) should be easy (just run the build inside a container or chroot or something). Compatibility is a bit tricky because those arches only exist in Debian Sid, which means we'd have to run the build inside a Sid container, and the Sid glibc would presumably impose rather stringent constraints on where you could run the resulting binaries. Maybe that could be worked around by using Debian Snapshot.