RFR: 8316961: Fallback implementations for 64-bit Atomic::{add,xchg} on 32-bit platforms [v3]

[Aleksey Shipilev]

> See the bug for rationale. Looks like there is enough infrastructure to achieve what we want without significant fan-out. I checked all `atomic_*.hpp` headers for unimplemented `PlatformAdd<8>` and `PlatformXchg<8>`, and only these seem to be affected.
> 
> Unfortunately, we cannot test these apart from the existing gtest.
> 
> Additional testing:
>  - [x] linux-x86-server-fastdebug, atomic tests pass
>  - [x] linux-arm-server-fastdebug, atomic tests pass (with #16269 applied)

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Protect 64-bit tests with supports_cx8()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16252/files
  - new: https://git.openjdk.org/jdk/pull/16252/files/f2c523ae..e7677da5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16252&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16252&range=01-02

  Stats: 15 lines in 1 file changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16252.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16252/head:pull/16252

PR: https://git.openjdk.org/jdk/pull/16252