Architecture support: s390

[ajdlinux]

I work on s390, I'd like to wire up s390 support.

This should be more straightforward than #23 - s390 has one main defconfig, is big-endian only, doesn't have a huge number of platforms, and in modern times is only supported in 64-bit mode - so I think I can just copy from the existing variables for x86_64/arm64.

I'm not aware of any good extensions for assembly syntax highlighting with the usual GNU syntax (as opposed to HLASM).