RFR: 8337511: Implement JEP 404: Generational Shenandoah (Experimental) [v12]

[William Kemper]

> This PR merges JEP 404, a generational mode for the Shenandoah garbage collector. The JEP can be viewed here: https://openjdk.org/jeps/404. We would like to target JDK24 with this PR.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 533 commits:

 - Merge remote-tracking branch 'jdk/master' into great-genshen-pr-redux
 - Problem list Shenandoah TestAllocOutOfMemory on ppc64
 - Merge remote-tracking branch 'shenandoah/master' into great-genshen-pr-redux
 - 8344985: GenShen: Refactor arraycopy barrier for generational mode
   
   Reviewed-by: shade
 - Remove incomplete test case
 - Merge remote-tracking branch 'shenandoah/master' into great-genshen-pr-redux
 - Merge
 - 8344779: GenShen: Consolidate shared heap region closures into one header
   
   Reviewed-by: kdnilsen, ysr
 - 8344797: GenShen: Update and rename confusing method
   
   Reviewed-by: ysr, kdnilsen
 - Merge remote-tracking branch 'shenandoah/master' into great-genshen-pr-redux
 - ... and 523 more: https://git.openjdk.org/jdk/compare/a80ccf2c...f29d741c

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Changes: https://git.openjdk.org/jdk/pull/21273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21273&range=11
  Stats: 22878 lines in 229 files changed: 21122 ins; 913 del; 843 mod
  Patch: https://git.openjdk.org/jdk/pull/21273.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21273/head:pull/21273

PR: https://git.openjdk.org/jdk/pull/21273