Skip unions and use alias_is_c_enu for AIX CI

[xingxue-ibm]

Description

This PR fixes the CI failures in issue #4870.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[tgross35]

LGTM, thanks!

I pushed with an updated commit description. We have a lot of CI, "Fix libc CI." isn't too specific :)

(for future reference, an aix: prefix in the commit message helps, I usually need to add it when making the changelog anyway)