Support building rustup for Windows arm64

[Alovchin91]

This PR adds support for building rustup for Windows arm64 using aarch64-pc-windows-msvc toolchain.

Things to note:

• CI templates are present but do not run builds on Windows arm64 yet
• Ring still does not support Windows arm64. See Windows arm64 support (aarch64-pc-windows-msvc) briansmith/ring#1167

See #2612 for discussion

[Alovchin91]

Test results: (see attachments)

All run under rustc 1.55.0-beta.3 (53fd98ca7 2021-07-31) installed by that same arm64 rustup 🙂

Note: for download tests I had to manually remove rustls-tls (requires ring) from Cargo.toml default features because cargo test doesn't seem to respect --no-default-features/--features flags when passed a -p parameter.

dump_testament.txt
cargo_test.txt
download_test.txt

[kinnison]

When you updated the CI actions, did you update the template and then run the script to regenerate the workflow files? If so then I need to poke the scripts. Otherwise this PR looks good. I'll check that over and then consider merging.

[Alovchin91]

When you updated the CI actions, did you update the template and then run the script to regenerate the workflow files?

Yep, that's what I did. Apparently I also need to mark those parameters in the "include" section with "skip-pr skip-master" otherwise they seem to be picked up by the CI.

[kinnison]

I think you must be right - it is over-executing on PRs for sure. Try adding the markers to the include section too please, yes.

[Alovchin91]

Sure! Do you think it makes sense to disable win-arm64 builds for now (since we seem to be making some good progress on *ring*: briansmith/ring#1339), or should we still enable them for stable or master, for example?

[kinnison]

I'm not sure if we're OK to release until ring is sorted, as such let's build on master, but not pr or stable.

[Alovchin91]

👍 Done. Also updated the README.

[Alovchin91]

Hi @kinnison Do you think this could be merged now? 🙂