It's now possible to natively compile a flutter app for
windows-arm64. Cross-compilation is not yet implemented.

Uses arm64 artifacts now available for Dart/Flutter.
Platform detection is based on Abi class, provided by Dart. Depending if
Dart is an arm64 or x64 binary, the Abi is set accordingly.
Initial bootstrap of dart artifacts (update_dart_sdk.ps1) is checking
PROCESSOR_ARCHITECTURE environment variable, which is the way to detect
host architecture on Windows.

This is available only for master channel (on other channels, it
fallbacks to windows-x64).

On windows-x64, it produces an x64 app. On windows-arm64, it produces an
arm64 app.