Allow test_is_broken for specific arches

I encountered this issue with argon2 and uom. argon2 had some failing tests exclusively on s390x. So I had to analyze the logs manually and disable all failing tests with #[cfg(arch(not="s390x"))]. For uom some arm tests are failing because of some weird edge case (integer overflow). Same solution.

It would be nice in the long run to pass some flags to the autopkgtest to skip the tests on those arches as only takes time to find the issue, write proper patches and upload a new revision. I don't know if this is technically possible.

I would style it like that:

[packages.lib]
test_is_broken = false
arch_test_is_broken = ["armel", "arm64"]