RFR: 8350898: Shenandoah: Eliminate final roots safepoint

[William Kemper]

Not clean. Conflicts with oop closure refactoring (8342588).

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Commit messages:
 - 8350898: Shenandoah: Eliminate final roots safepoint

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/179/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=179&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350898
  Stats: 296 lines in 14 files changed: 199 ins; 47 del; 50 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/179.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/179/head:pull/179

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/179