RFR: 8359394: Shenandoah: GC cause cleanup

[Stefan Karlsson]

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

> We can remove `GCCause::_no_cause_specified` and need to add a case for `_shenandoah_humongous_allocation_failure` in `GCCause::to_string`. We are also missing `_shenandoah_humongous_allocation_failure` in the `SA` analog.

I'm OK with the shared GC changes if you change the title of JBS issue or split it out the shared parts into a separate issue.

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PR Review: https://git.openjdk.org/jdk/pull/25789#pullrequestreview-2930994011