[clang] Use the XL pragma pack semantics on z/OS (PR #111053)

[Abhina Sree via cfe-commits]

https://github.com/abhina-sree approved this pull request.

LGTM, can we add [SystemZ][z/OS] in the title to make it consistent with other PRs

https://github.com/llvm/llvm-project/pull/111053