[jdk25u] RFR: 8365571: GenShen: PLAB promotions may remain disabled for evacuation threads

[William Kemper]

Clean backport. Fixes performance issue with generational mode for Shenandoah.

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Commit messages:
 - Backport 08db4b99622e488558dd7987c34f1c515fa30426

Changes: https://git.openjdk.org/jdk25u/pull/100/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=100&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365571
  Stats: 24 lines in 2 files changed: 5 ins; 15 del; 4 mod
  Patch: https://git.openjdk.org/jdk25u/pull/100.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/100/head:pull/100

PR: https://git.openjdk.org/jdk25u/pull/100