RFR: 8350898: Shenandoah: Eliminate final roots safepoint [v2]

[William Kemper]

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

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'shenandoah-jdk21u-master' into backport-shenandoah-jdk21u-8350898
 - 8350898: Shenandoah: Eliminate final roots safepoint
   
   Reviewed-by: rkennke, kdnilsen, cslucas

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah-jdk21u/pull/179/files
  - new: https://git.openjdk.org/shenandoah-jdk21u/pull/179/files/fd062d26..a8d6e133

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=179&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=179&range=00-01

  Stats: 846 lines in 29 files changed: 553 ins; 94 del; 199 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