Update x64-software-conventions.md

[sigatrev]

Add link to __vectorcall documentation

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[learn-build-service-prod[bot]]

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[Court72]

@TylerMSFT

Can you review the proposed changes?

When the changes are ready for publication, add a #sign-off comment to signal that the PR is ready for the review team to merge.

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

[learn-build-service-prod[bot]]

Learn Build status updates of commit c297b33:

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For any questions, please:

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[TylerMSFT]

#sign-off