RFR: 8344640: GenShen: Reuse existing card mark barrier function when dropping references

[William Kemper]

When GenShen drops references during concurrent weak reference processing, it manipulates pointers in a list that is threaded through java objects and so must go through the card mark barrier. This change has it reuse an existing function for this purpose.

## Testing

Ran this through internal pipelines: Dacapo, Extremem, SpecJBB2015, SpecJVM2008, Heapothesys (with and without GC stress flags).

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into drop-with-card-barrier
 - Use card barrier when dropping reference

Changes: https://git.openjdk.org/shenandoah/pull/547/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=547&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344640
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/547.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/547/head:pull/547

PR: https://git.openjdk.org/shenandoah/pull/547