RFR: 8306321: Add accessor for top of PLAB

[William Kemper]

The generational mode for Shenandoah needs to know the top of a PLAB when it is retired.

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Commit messages:
 - Add accessor for top of PLAB

Changes: https://git.openjdk.org/jdk/pull/13516/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306321
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13516.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13516/head:pull/13516

PR: https://git.openjdk.org/jdk/pull/13516