Fix detection of aarch64 host on Windows

[arlosi]

rustup does not currently auto-detect aarch64-pc-windows-msvc hosts. Instead x86_64-pc-windows-msvc is used because GetNativeSystemInfo does not support detection of aarch64 hosts.

From the docs:

If the function is called from an x86 or x64 application running on a 64-bit system that does not have an Intel64 or x64 processor (such as ARM64), it will return information as if the system is x86 only if x86 emulation is supported (or x64 if x64 emulation is also supported).

The alternative function used in this change IsWow64Process2 supports detecting aarch64 hosts.

[arlosi]

I should note that IsWow64Process2 is only available on Windows 10 or newer. Is that acceptable? I couldn't find the minimum requirements for rustup.

If we need to go back further, I can do runtime detection of whether the function is available.

[ChrisDenton]

Note that Rust itself currently supports Windows 7+. Presumably rustup should attempt to do the same unless the maintenance burden gets too high.

[arlosi]

I've updated the PR to use runtime detection of IsWow64Process2 with a fallback to the original method using GetNativeSystemInfo