add s390x support

[cgwalters]

Right now we're just missing s390x, which adds messy architecture conditionals to our code.

The way zipl works is somewhat UKI-like as far as I understand things in that it needs to be run for each kernel update, and it merges together the kernel/initramfs. The ostree/zipl glue is basically just running zipl after an update to the BLS configs.

So I think the bootupd support for s390x is basically just "detect and no-op" effectively - almost.

Except for one detail: on the first install to a disk, there's a magic command that needs to be run called "chreipl".

Also xref https://github.com/coreos/coreos-installer/blob/c4ed04156ae21029e84acc3cdf51493dc1fa8b8b/src/s390x/README.md?plain=1#L11