RFR: 8308500: ZStatSubPhase::register_start should not call register_gc_phase_start if ZAbort::should_abort()

[Axel Boldt-Christmas]

`ZStatSubPhase::register_start` should not call `register_gc_phase_start` if `ZAbort::should_abort()` is true. This will cause an unbalanced push and pop behaviour of the phase stack as `ZStatSubPhase::register_end` stops popping (and sending events) after the aborting has started. This will create an issue if more subsequent sub-phases are added in-between two abort points as the phase stack may overflow. 

Simply avoid pushing new phases when aborting has started solves this issue.

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Commit messages:
 - 8308500: ZStatSubPhase::register_start should not call register_gc_phase_start if ZAbort::should_abort()

Changes: https://git.openjdk.org/jdk/pull/14075/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14075&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308500
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14075.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14075/head:pull/14075

PR: https://git.openjdk.org/jdk/pull/14075