s390x accepted as release architecture

Yay, so we made it: s390x got added as a release architecture. What this means:

• If the package already exists on that architecture and fails to build, that issue is deemed RC. You can debug the issue on the porterbox (zelenka), chroot sid_s390x. Build dependencies are installed by a team, just follow the request guidelines.
• If your package is not installable on s390x, it will not migrate to testing anymore. So if a package does not exist on s390x, you need to make sure not to generate a dependency on it or prevent your package from building on s390x (by e.g. build-depending on a package that's not available on s390x or by getting it into Packages-arch-specific).

This will also help other 64bit big-endian ports (like powerpc64 and sparc64) to enter the archive more easily, as most issues left are indeed related to endianness, not to specialities of the System z hardware.

Many thanks go to Aurélien Jarno, without whom this would not have been possible. I also want to take this opportunity to thank all our s390(x) machine sponsors: ZIVIT, IIC@KIT and Marist College. There are not many mainframe owners who let free software projects work on their machines.