ICU-22293 Add bot for gcc and -Werror

[FrankYFTang]

Checklist

• [X] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22293
• [X] Required: The PR title must be prefixed with a JIRA Issue number.
• [X] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [X] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

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