[clang] [SystemZ][z/OS] Add z/OS customization file (PR #111182)

[Sean Perry via cfe-commits]

perry-ca wrote:

build failures were from time outs uploading to the artifactory.  Is unrelated to change.
```
2025-01-06 21:33:11 ERROR  Error uploading artifact states: PUT https://agent.buildkite.com/v3/jobs/01943cd5-d77e-4011-a1f4-08aabba24f73/artifacts: 504 Gateway Time-out
--
  | fatal: failed to upload artifacts: uploading artifacts: errors uploading artifacts: [PUT https://agent.buildkite.com/v3/jobs/01943cd5-d77e-4011-a1f4-08aabba24f73/artifacts: 504 Gateway Time-out]
```

https://github.com/llvm/llvm-project/pull/111182