Add support for native arm64 Windows

[rmartin16]

What is the problem or limitation you are having?

Toga cannot currently be used with native arm64 Python on Windows.

However, this issue isn't really tracking work for Toga as much as it is for Python.NET. Currently, Python.NET does not provide a CLR loader for arm64; instead, it tries to load the CLR for x86-64. While Windows on ARM can load x86-64 DLLs, it cannot do so in an arm64 Python process.

Therefore, Toga can only be used on arm64 Windows via an x86-64 Python running in emulation; arguably, though, this is sufficient for most uses of Toga.

Describe the solution you'd like

Once Python.NET adds support for Windows on ARM, ensure Toga can use it with arm64 Python.

Describe alternatives you've considered

Use x86-64 Python on arm64 Windows.

Additional context

• Add support for Windows on ARM briefcase#1887
• Add support for Windows ARM64 pythonnet/pythonnet#1885