configure: Enable ARM64 single-toolchain build on Windows

When target arch is arm64, set_msvs_configuration platfrom in order to
generate proper .sln/.vcxproj files for cross-compiling node modules
to Windows on Arm, 64-bit. To use, run `set npm_config_arch=arm64`
on the command prompt before `npm install`.

PR-URL: #1678

lib/configure.js

@@ -134,6 +134,9 @@ function configure (gyp, argv, callback) {
 
     // set the target_arch variable
     variables.target_arch = gyp.opts.arch || process.arch || 'ia32'
+    if (variables.target_arch == 'arm64') {
+      defaults['msvs_configuration_platform'] = 'ARM64'
+    }
 
     // set the node development directory
     variables.nodedir = nodeDir