DRTVWR-418: Provide Darwin_i686_Manifest alias to Darwin_i386_Manifest.

It's never been clear to me why Macs tend to refer to 32-bit Intel processors
as i386 when other platforms tend to refer to them as i686. New CMake logic to
derive ARCH from ADDRESS_SIZE produces i686. Give viewer_manifest.py a
Darwin_i686_Manifest class alias so it continues to work when arch is passed
as i686 as well as i386.

indra/newview/viewer_manifest.py

@@ -1049,6 +1049,10 @@ def package_finish(self):
         self.package_file = finalname
         self.remove(sparsename)
 
+class Darwin_i686_Manifest(Darwin_i386_Manifest):
+    """alias in case arch is passed as i686 instead of i386"""
+    pass
+
 class LinuxManifest(ViewerManifest):
     def construct(self):
         super(LinuxManifest, self).construct()