cpu/clocktest should not run on s390x

[bladernr]

Bug Description

The clocktest script doesn't work on Z due to issues with sched_setaffinity. Having talked with Frank before we don't think this is really necessary as a test case on IBM Z anyway so it's safe to just skip it on s390x arch systems.

To Reproduce

Run cpu_clocktest on an LPAR or zVM guest on the Z

Environment

IBM Z running Lunar (has been seen on other releases too, test is inconsequential on Z and can be skipped)

Relevant log output

sched_setaffinity: Invalid argument
Testing for clock jitter on 282 cpus

Additional context

No response

[pieqq]

The job in question is in the base provider:

https://github.com/canonical/checkbox/blob/main/providers/base/units/cpu/jobs.pxu#L126-L135

It is currently executed for any architecture, but this can be tailored using the requires field of the job unit and the cpuinfo resource job, similar to the cpu/cstates job for instance.