Jenkinsfile: skip ppc64le and s390x by default on pull requests

[thaJeztah]

This changes CI to skip these platforms by default. The ppc64le and s390x
machines are "pet machines", configuration may be outdated, and these
machines are known to be flaky.

Building and verifying packages for these platforms is being handed
over to the IBM team.

We can still run these platforms for specific pull requests by selecting
the checkboxes.

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[tianon]

😢 but also fully understandable and +1

LGTM

[thaJeztah]

😢 but also fully understandable and +1

We could re-enable them at some point (there definitely are some things to look into (e.g. need to have another look into #42231)

Current reality though, was that most of the time we would either ignore the failures, "skip" these ("build with parameters"), or repeatedly run Jenkins "until it goes green", which was a waste of time IMO.

We could probably add a stage that just builds the binaries (to make sure it "compiles"), or make sure that our cross target also builds for these

[tianon]

Yeah, totally sane -- maybe instead of doing them on every PR we have a separate "master only" build that runs them so they're still being run and folks can look at them if they want to help contribute to making sure they're supported but they aren't holding us up on PRs? hahahahaha, that's literally what this PR does. I'm so good. 🤦

(100% on board with at least a compilation check for any architecture we "support" even if that support only amounts to "it builds")