Add {linux,darwin}/aarch64 to cross-compile platforms.

[tmds]

Fixes #1077.

@adambkaplan ptal.

[openshift-ci]

Hi @tmds. Thanks for your PR.

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[adambkaplan]

/ok-to-test

[tmds]

@adambkaplan CI looks happy. Is this good to merge?

[adambkaplan]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adambkaplan, tmds

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[adambkaplan]

@tmds looks good, thank you for the PR!