RFR: 8367013: Add Atomic<T> to package/replace idiom of volatile var plus AtomicAccess:: operations [v5]

[Kim Barrett]

> Please review this change that adds the type Atomic<T>, to use as the type
> of a variable that is accessed (including writes) concurrently by multiple
> threads. This is intended to replace (most) uses of the current HotSpot idiom
> of declaring a variable volatile and accessing that variable using functions
> from the AtomicAccess class.
> https://github.com/openjdk/jdk/blame/528f93f8cb9f1fb9c19f31ab80c8a546f47beed2/doc/hotspot-style.md#L138-L147
> 
> This change replaces https://github.com/openjdk/jdk/pull/27462. Differences are
> 
> * Substantially restructured `Atomic<T>`, to be IDE friendly. It's
> operationally the same, with the same API, hence uses and gtests didn't need
> to change in that respect. Thanks to @stefank for raising this issue, and for
> some suggestions toward improvements.
> 
> * Changed how fetch_then_set for atomic translated types is handled, to avoid
> having the function there at all if it isn't usable, rather than just removing
> it via SFINAE, leaving an empty overload set.
> 
> * Added more gtests.
> 
> Testing: mach5 tier1-6, GHA sanity tests

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

  rename relaxed_store => store_relaxed

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27539/files
  - new: https://git.openjdk.org/jdk/pull/27539/files/934a4ed2..f7e2d950

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27539&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27539&range=03-04

  Stats: 52 lines in 8 files changed: 0 ins; 0 del; 52 mod
  Patch: https://git.openjdk.org/jdk/pull/27539.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27539/head:pull/27539

PR: https://git.openjdk.org/jdk/pull/27539