Update Dockerfile to use gcc-toolset-14 and fix test case failures on power (ppc64le)

[bhagyashrigai]

Purpose

This PR updates the ppc64le Dockerfile to use gcc-toolset-14 instead of gcc-toolset-13 and includes additional environment fixes required for successful builds. The update resolves multiple test case failures observed during vLLM and dependency builds on the ppc64le platform.

Key Changes

• Updated all Dockerfile stages from gcc-toolset-13 → gcc-toolset-14.
• Updated compiler activation paths (source /opt/rh/gcc-toolset-14/enable).
• Exported LLVM_CONFIG and updated PATH to ensure correct LLVM detection.
• Added missing dependency (libicu) for ppc64le build stability.
• Updated platform selectors in requirements/common.txt to include:platform_machine == "ppc64le" for llguidance and xgrammar

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.