Visual Studio: Improve NMake Makefiles for ARM64 Windows builds (master branch)

Hi,

This MR attempts to improve the NMake Makefiles for AR64 Windows builds by:

• Allow linking ARM64 binaries (by checking that we are building for ARM64 and thus pass in /machine:arm64 during linking, which is needed for the Visual Studio cross-compiler that we will be using in this case.
• Improve the header search path: make sure that we are looking for headers in $(PREFIX)\include.

With blessings, thank you!