s390x: Fix PyArrow build and add CPU test script for Buildkite CI

[Nash-123]

Summary

This PR adds IBM Z (s390x) architecture support to vLLM by implementing Buildkite CI testing.

Changes

.buildkite/run-cpu-test-s390x.sh

• Added new CI script for s390x-specific testing workflow

docker/Dockerfile.s390x

• Fixed PyArrow compilation failures
• Resolved huggingface-hub[hf-xet] incompatibility by building from source

Notes

• Encountered the same issue with huggingface-hub[hf_xet] on s390x as reported for ppc64le in [RFC]: Is huggingface-cli[hf_xet] needed for vllm build? #16012
• This PR addresses it by building hf-xet from source on s390x

Testing

Tested the job using a connected s390x Buildkite agent on IBM Cloud
Triggered pipeline from vLLM community Buildkite setup
Sample Build: CI Build #16786
PR Buildkite-CI:- vllm-project/buildkite-ci#84

c.c: @khluu

[github-actions]

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