ci: Test powerpc64-unknown-linux-musl in tier 2

[Gelbpunkt]

Description

Since 1.95.0, this is now a tier 2 target.

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

[Gelbpunkt]

This is going to be a bit of fun, since the Ubuntu cross compilation toolchain is for ELFv1. We'll have to compile libgcc ourselves or just start using the Rust CI images that contain ready-to-use toolchains.