RFR: 8359394: Shenandoah: GC cause cleanup

[William Kemper]

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.

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Commit messages:
 - Fix typo
 - Clean up/fix gc causes for Shenandoah

Changes: https://git.openjdk.org/jdk/pull/25789/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25789&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359394
  Stats: 18 lines in 5 files changed: 5 ins; 6 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/25789.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25789/head:pull/25789

PR: https://git.openjdk.org/jdk/pull/25789