RFR: Update remembered set during old generation mark

[earthling-amzn]

This change is to support the concurrent remembered set scanning work which is in progress.

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Commit messages:
 - Update remembered set during old generation mark

Changes: https://git.openjdk.java.net/shenandoah/pull/31/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=31&range=00
  Stats: 14 lines in 3 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/31.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/31/head:pull/31

PR: https://git.openjdk.java.net/shenandoah/pull/31