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

[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
 - [ ] linux-arm-server-fastdebug, atomic tests pass

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Commit messages:
 - Work

Changes: https://git.openjdk.org/jdk/pull/16252/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316961
  Stats: 93 lines in 5 files changed: 82 ins; 8 del; 3 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