Update the ARM64EC-related predefined macros

[mcfi]

No description provided.

[PRMerger9]

@mcfi : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[opbld30]

Docs Build status updates of commit 21ef818:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/preprocessor/predefined-macros.md	✅Succeeded

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[Jak-MS]

@corob-msft

Can you review this PR?

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#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[colin-home]

LGTM. Thanks!

[opbld33]

Docs Build status updates of commit 21ef818:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/preprocessor/predefined-macros.md	✅Succeeded

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel