RFR: 8359394: GC cause cleanup

[Y. Srinivas Ramakrishna]

On Thu, 12 Jun 2025 23:42:41 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Remove `GCCause::_no_cause_specified` (only referenced by Shenandoah) and add a case for `_shenandoah_humongous_allocation_failure` in `GCCause::to_string` and the `SA` analog.

Thanks for changing the title/summary to broaden scope.

Re-approved; 🚢

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Marked as reviewed by ysr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25789#pullrequestreview-2932895948