RFR: Do not visit gc threads during shutdown

[William Kemper]

Discovered a strange crash during shutdown while working on jtreg tests.

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Commit messages:
 - Do not visit gc threads during shutdown

Changes: https://git.openjdk.org/shenandoah/pull/228/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=228&range=00
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/228.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/228/head:pull/228

PR: https://git.openjdk.org/shenandoah/pull/228