Exclude i386 for Charm build recipes

[Haw Loeung]

Hi,

Per build failure log[1], `charmcraft` doesn't support `i386`:

```
error: snap "charmcraft" is not available on stable for this architecture
       (i386) but exists on other architectures (amd64, arm64, armhf, ppc64el,
       s390x).
Install failed
```

Also, per build failure log[2], `charm-tools` doesn't support `s390x`:

```
Parts processing error: Error installing snap 'charm' from channel '2.x/stable'.
```

I think we should exclude these two architectures from charm builds.

[1]https://launchpadlibrarian.net/685614874/buildlog_charm_ubuntu_bionic_i386_content-cache_BUILDING.txt.gz
[2]https://launchpadlibrarian.net/685625973/buildlog_charm_ubuntu_focal_s390x_content-cache_BUILDING.txt.gz

[Colin Watson (cjwatson)]

i386 I can see (though IIRC it's kind of tricky to arrange neatly which is why we haven't), but I don't understand the logic that says we should drop s390x builds from all charm recipes just because the reactive machinery doesn't support it. If we were going to do that, wouldn't it be better to drop it only from reactive charms? (Though it may be kind of tricky to detect that reliably; I suppose we could check for layer.yaml or something.)

[Haw Loeung (hloeung)]

Oh yeah, totally forgot about `charm-tools` only being used to build reactive charms.