[llvm] [JITLINK] Disable ELF_ppc64_relocations.s on SystemZ host (PR #168939)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand approved this pull request.

LGTM.  Merging this to make the SystemZ build bots green again.

https://github.com/llvm/llvm-project/pull/168939