RFR: Also register filler object for retired gclabs

[William Kemper]

Shenandoah verify will complain that the objects created when gclabs are retired are not properly registered. We already have code to register objects when the promotion labs are retired, this change extends it to also cover other gclabs.

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Commit messages:
 - Also register filler object for retired gclabs

Changes: https://git.openjdk.java.net/shenandoah/pull/74/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=74&range=00
  Stats: 15 lines in 1 file changed: 4 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/74.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/74/head:pull/74

PR: https://git.openjdk.java.net/shenandoah/pull/74