RFR: 8337511: Implement JEP 404: Generational Shenandoah (Experimental) [v11]

[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 incrementally with two additional commits since the last revision:

 - Merge remote-tracking branch 'shenandoah/master' into great-genshen-pr-redux
 - 8344985: GenShen: Refactor arraycopy barrier for generational mode
   
   Reviewed-by: shade

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21273/files
  - new: https://git.openjdk.org/jdk/pull/21273/files/381af0cd..08b59bc3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21273&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21273&range=09-10

  Stats: 14 lines in 1 file changed: 6 ins; 7 del; 1 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