RFR: 8333109: GenShen: Factor generational mode out of gc helpers (redo)

[William Kemper]

Clean backport

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Commit messages:
 - Backport 840c1825f38a11485243a648b171b1ba5ec78de5

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/54/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=54&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333109
  Stats: 345 lines in 6 files changed: 132 ins; 140 del; 73 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/54.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/54/head:pull/54

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/54