RFR: 8374328: Convert simple AtomicAccess uses in gc/shared to use Atomic<T> [v2]

[Kim Barrett]

> Please review these fairly simple changes to code in gc/shared to use
> `Atomic<T>` instead of directly using `AtomicAccess`.
> 
> This change doesn't eliminate all remaining direct uses of `AtomicAccess` in
> that directory; there are still a few uses that seem less simple, and maybe
> some that should stay with direct `AtomicAccess`.
> 
> The changes are grouped into a series of commits, as a potential aid to reviewers.
> 
> Testing: mach5 tier1-5

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  update copyrights

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28987/files
  - new: https://git.openjdk.org/jdk/pull/28987/files/0abdb185..ad121490

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28987&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28987&range=00-01

  Stats: 12 lines in 12 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/28987.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28987/head:pull/28987

PR: https://git.openjdk.org/jdk/pull/28987