RFR: 8321815: Shenandoah: gc state should be synchronized to java threads only once per safepoint [v4]

[William Kemper]

> Presently, every change to the gc change is distributed to all java threads. The gc state is only changed on safepoints. For applications with a very large number of java threads this may increase time on the safepoint. It would be better to synchronize the gc state only once per safepoint.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Initialize new _gc_state_changed member.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17112/files
  - new: https://git.openjdk.org/jdk/pull/17112/files/d1a4a692..eb6f3fdf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17112&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17112&range=02-03

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17112.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17112/head:pull/17112

PR: https://git.openjdk.org/jdk/pull/17112