RFR: 8320582: Zero: Misplaced CX8 enablement flag

[Aleksey Shipilev]

When doing [JDK-8319777](https://bugs.openjdk.org/browse/JDK-8319777), I misplaced the `_supports_cx8 = true` flag setting in the method that is only called when CPU features are polled from perf counter code. We need to move the check to a proper place. [JDK-8318776](https://github.com/openjdk/jdk/pull/16625/files) would catch fire without this.

Additional testing (redoing JDK-8319777 testing):
 - [x] Linux arm Zero fastdebug now builds fine with JDK-8318776 fix
 - [ ] Linux x86_32 Zero release; jcstress
 - [ ] Linux x86_32 Zero fastdebug, `compiler/unsafe java/lang/invoke/VarHandles`
 - [ ] Linux x86_32 Zero fastdebug, bootcycle-images

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16779/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16779&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320582
  Stats: 12 lines in 1 file changed: 6 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16779.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16779/head:pull/16779

PR: https://git.openjdk.org/jdk/pull/16779