RFR: 8320582: Zero: Misplaced CX8 enablement flag

[David Holmes]

On Wed, 22 Nov 2023 10:33:24 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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

It is a bit unclear exactly where the right place is in the sense that I'd expect the other variables, like `_no_of_cores`, to be initialized at the same time. But ok.

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16779#pullrequestreview-1744276114