[Windows Arm64] Update tool to target Arm64 on Arm64 hosts

[loic-sharma]

Background

See: https://flutter.dev/go/windows-arm64

Once #129805 lands, Arm64 hosts will be able to build x64 executables. These executables run using x64 emulation.

Solution

1. Add Windows Arm64 to the HostPlatform and TargetPlatform enums
2. Update the Flutter Windows plugin templates to show how to use the target architecture to bundle pre-built libraries
3. If on Arm64 host and the "beta" or "stable" release channels, produce a warning that an x64 executable will be built.
4. If on Arm64 host and the "master" release channel,
   1. Use Abi.current() to determine the target architecture
   2. Download Windows Arm64 artifacts
5. Update Windows Arm64 devicelab tests: build, run, benchmark

[loic-sharma]

One this work lands, we should clean up the packages repo tooling:

1. https://github.com/flutter/packages/pull/4826/files#diff-b525444e626a654538177afff3418a5da77038d43a58e3b52118523f87e9b7bfR316-R339
2. https://github.com/flutter/packages/pull/4826/files#diff-bb0d4d38717be1c6c0cae870a81c95d238c7106871b1437a083a5c1debe0d469R592-R601