RFR: 8319777: Zero: Support 8-byte cmpxchg

[Aleksey Shipilev]

See related discussion in [JDK-8318776](https://bugs.openjdk.org/browse/JDK-8318776) that targets to require `supports_cx8()` unconditionally.

I think we can claim Zero is `supports_cx8() == true`, because we have enough fallbacks for 8-byte CASes to work. Note that some code already reaches for these without checking for `supports_cx8()`, so the proverbial horses have already left the barn.

I ran tests with [JDK-8319883](https://bugs.openjdk.org/browse/JDK-8319883) applied to fix known problems with x86_32 Zero.

Additional testing:
 - [x] Linux x86_32 Zero release; jcstress
 - [x] Linux x86_32 Zero fastdebug, `compiler/unsafe java/lang/invoke/VarHandles`
 - [x] Linux x86_32 Zero fastdebug, bootcycle-images

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16614/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16614&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319777
  Stats: 7 lines in 2 files changed: 5 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16614/head:pull/16614

PR: https://git.openjdk.org/jdk/pull/16614