Build x86 and x64 API binaries on AppVeyor

- Identify the platform in the archive filenames to avoid name
  conflicts.
- Run x64 builds first to fail faster.
- Only build the API and its tests during x64 builds.
- Add a separate GitHub deployment config for the x64 build, as
  using the same config for both will fail when most artifacts are
  missing for x64. It's not great that everything needs to be
  given twice, but there's no "add file to release" option.

scripts/archive.js

@@ -221,6 +221,8 @@ function createMetadataValidatorArchive(rootPath, binaryPath, tempPath, destPath
 function getFilenameSuffix(label, gitDescription) {
   if (label) {
     return `${gitDescription}_${label}`;
+  } else if (process.env.APPVEYOR) {
+    return `${gitDescription}_${process.env.PLATFORM}`;
   }
 
   return `${gitDescription}`;