RFR: 8341097: GHA: Demote Mac x86 jobs to build only

[Kim Barrett]

On Mon, 30 Sep 2024 08:36:38 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See the discussion in the bug. I think we can stop testing Mac x86 in GHA, leaving only the build jobs.
> 
> Additional testing:
>  - [x] GHA passes

This change seems premature.  Various companies (Oracle, SAP, others?) are still
supporting macos-x64 on mainline.  Removing this seems to be just asking for more
frequent backouts due to x64-specific changes that aren't compatible with clang.

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Changes requested by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21257#pullrequestreview-2337070818