Enable cross compilation for windows-arm64

[p-b-o]

This is available only for master channel.

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Added option --target-platform {windows-x64, windows-arm64} for flutter build windows command.

Without this option, it produces an x64 app on windows-x64, and arm64 app on windows-arm64.

This was successfully tested (on windows-x64 and windows-arm64) with: $ flutter create app
$ cd app
$ flutter build windows
$ flutter build windows --target-platform windows-x64 $ flutter build windows --target-platform windows-arm64

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[flutter-dashboard]

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[p-b-o]

This PR is the succession of #141930.
However, it can't be upstreamed in this state.

We have been discussing in a previous PR (#137618) that ephemeral folder should be specialized per architecture. For now, cross compilation would work by reusing same ephemeral folder, but it might break one day.

Another issue as well, is how to download flutter artifacts only for one platform. We didn't identify a correct way to do it in our discussions, and it's an open problem.

I might come back to work on this later, but for now, this is lower priority. So if someone is willing to work on this, you're more than welcome! Don't hesitate to ping me for review or questions.