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

[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.

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

Commit messages:
 - Fix whitespace
 - Sync gc state with threads on VM thread
 - Synchronize gc state with java threads once, when exiting safepoint

Changes: https://git.openjdk.org/jdk/pull/17112/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17112&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321815
  Stats: 25 lines in 5 files changed: 17 ins; 1 del; 7 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