Enable building for Windows ARM64

[JunielKatarn]

Allows building natively for Windows/ARM64 devices (Surface Pro X, Volterra, etc).

[ArminiusTux]

Greetings, just wanted to drop my experience in compiling the MP portion on WinARM64.
The above "__rdtsc workaround" really gets the other two executables, openjk.x86_64.exe & openjkded.x86_64.exe compiled, but they crash outright.

The SP.exe is not exactly rock solid, but absolutely playable.

[ArminiusTux]

Is there any way to kick this can / pull request further down the road?

[Razish]

I don't love having vendored versions for everything except arm64-windows.
I think my preference would be vendored versions for all, with optional sourcing from (system) package manager. I also don't love how much bloat and churn that entails. Maybe arm64-windows is a special-enough case to push for vcpkg.

This isn't a blocker, just provoking discussion so we can form a stance here intentionally.