Add ARM64 to MSBuild /Platform logic

PR-URL: #1655
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: João Reis <reis@janeasystems.com>

lib/build.js

@@ -200,9 +200,13 @@ function build (gyp, argv, callback) {
 
     // Specify the build type, Release by default
     if (win) {
+      // Convert .gypi config target_arch to MSBuild /Platform
+      // Since there are many ways to state '32-bit Intel', default to it.
+      // N.B. msbuild's Condition string equality tests are case-insensitive.
       var archLower = arch.toLowerCase()
       var p = archLower === 'x64' ? 'x64' :
-              (archLower === 'arm' ? 'ARM' : 'Win32')
+              (archLower === 'arm' ? 'ARM' :
+              (archLower === 'arm64' ? 'ARM64' : 'Win32'))
       argv.push('/p:Configuration=' + buildType + ';Platform=' + p)
       if (jobs) {
         var j = parseInt(jobs, 10)