Enable ARM64 builds using release product tools

Enable ARM64 builds using release product tools

Remove support for specifying the toolset directory for arm64,
which was used to point to an internal toolset.

Building for arm64 now works just like the other platforms, e.g.
invoke `build arm64`.

The requirements:
. Visual Studio 2017 Update 4 or later, with ARM64 toolset installed
. Windows SDK 10.0.17134.0 or later
. CMake 3.10 or later