For non-winrt 64-bit Windows builds

[aprothman]

Use the 64-bit native compiler rather than the 32 to 64 cross compiler.
Changed the toolchain batch file to point relevant options and paths to amd64

[intoxopox]

Has anyone reviewed this for consideration?

[hughsando]

The scripts should be using 'host_arch' now, eg https://github.com/HaxeFoundation/hxcpp/blob/master/toolchain/msvc-arm64-setup.bat#L16
Is this PR still needed?

[aprothman]

https://github.com/HaxeFoundation/hxcpp/blob/master/toolchain/msvc64-setup.bat#L3
In the Windows x64 case, the batch file that should be run is vcvars64.bat. vcvarsx86_amd64.bat sets up the 64-bit cross compiler meant to be run from 32-bit operating systems to build 64-bit applications. It forces the build process to be run from the WOW64 subsystem. The results should be binarily compatible, but in practice they are different. Of course, we can also run into the 32-bit process memory limits for large builds while using the cross compiler.