RFR: 8320969: Shenandoah: Enforce stable number of GC workers [v2]

[Aleksey Shipilev]

> See bug for rationale.
> 
> Additional testing:
>  - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`
>  - [x] Linux x86_64 server fastdebug, `tier{1,2,3}` with `-XX:+UseShenandoahGC`
>  - [x] Linux AArch64 server fastdebug, `tier{1,2,3}` with `-XX:+UseShenandoahGC`

Aleksey Shipilev 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 four additional commits since the last revision:

 - Merge branch 'master' into JDK-8320969-shenandoah-stable-gc-workers
 - Reinstate the .cpp policy code
 - Merge branch 'master' into JDK-8320969-shenandoah-stable-gc-workers
 - No dynamic GC workers

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16878/files
  - new: https://git.openjdk.org/jdk/pull/16878/files/56d597be..e5826b15

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16878&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16878&range=00-01

  Stats: 85136 lines in 1765 files changed: 39360 ins; 39156 del; 6620 mod
  Patch: https://git.openjdk.org/jdk/pull/16878.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16878/head:pull/16878

PR: https://git.openjdk.org/jdk/pull/16878