[native_toolchain_c] Support native windows-arm64 MSVC toolchain

[pbo-linaro]

#167 introduce support for MSVC arm64 toolchain.

For now, it uses always x64 -> arm64 toolchain, that is guaranteed to be available on both windows-x64 and windows-arm64.
We could optimize further and use native arm64 -> arm64 toolchain when being on windows-arm64.

[dcharkes]

#167 Introduces recognizing the x64->arm64 toolchain, but it does not exercise it in unit tests. The place where we should add the cross compilation unit tests:

• https://github.com/dart-lang/native/blob/main/pkgs/native_toolchain_c/test/cbuilder/cbuilder_cross_windows_host_test.dart

(A local try for me didn't work because vc 2022 didn't include Hostx64/arm64/. Possibly newer msvc's include it, or I need to install it.)