Build and release aarch64 android binaries.

[pietro]

Adding Android Aarch64 to the buildbots was discussed on https://internals.rust-lang.org/t/suggestion-make-real-world-tier-1-platforms-at-least-tier-2/3492

Also e5ac28a has the commit log "Build and release i686+aarch64 android binaries" but it doesn't add Aarch64.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

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[alexcrichton]

Thanks @pietro! I'm finishing up running some tests locally and I should hopefully ship this soon to production.

[pietro]

Thank you @alexcrichton without your pointers I'd be lost on master.cfg.