RFR: 8374686: ZGC: Convert zMarkTerminate to use Atomic<T>

[Axel Boldt-Christmas]

Convert usages of AtomicAccess to Atomic<T> in zMarkTerminate.

There are a few things that potentially should be changed here. 

First the `_nworkers` variable was accessed through the AtomicAccess API in a handful of places. In this patch I changed it to always be an Atomic. However, this variable is only ever read concurrently. It only gets modified before we schedule a task on the workers (either for a new task, or after all workers have stopped working because we are resizing the work gang size).  So potentially this could just be changed to a uint with plain loads and stores.

The other variables have mutual exclusion when updated, but are read concurrently without synchronisation. In this patch I changed the volatile load + sub/add + atomic relaxed store [+ volatile load] into a simple atomic relaxed add/sub [and fetch]. The main reason being that I thought the code became less verbose, easier to read and understand. 

Testing:
 * GHA
 * HotSpot GC tests with ZGC linux x64 and linux aarch64

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Commit messages:
 - ZGC: Convert zMarkTerminate to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29246/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29246&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374686
  Stats: 29 lines in 2 files changed: 1 ins; 3 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/29246.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29246/head:pull/29246

PR: https://git.openjdk.org/jdk/pull/29246