RFR: 8333930: GenShen: Check for cancellation of old mark after final mark

[William Kemper]

Clean backport

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Commit messages:
 - Backport e3c3098a682be9041045140cb89bdda43f4bff30

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/56/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=56&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333930
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/56.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/56/head:pull/56

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/56