change the build directory for outside files of project

This changing will introduce:

1. When run 'rake clean', the object files of gem is not removed. To reduce the time when rebuild the project.
2. Across multiple projects that use the same gem, this changing will share the object files.
3. When update RubyMotion with 'motion update', it will remove the object files to clean up disk space.

lib/motion/project/xcode_config.rb

@@ -209,16 +209,18 @@ EOS
       end
     end
 
-    def archs
-      @archs ||= begin
-        h = {}
-        platforms.each do |platform|
-          h[platform] = Dir.glob(File.join(datadir, platform, '*.bc')).map do |path|
-            path.scan(/kernel-(.+).bc$/)[0][0]
-          end
+    def default_archs
+      h = {}
+      platforms.each do |platform|
+        h[platform] = Dir.glob(File.join(datadir, platform, '*.bc')).map do |path|
+          path.scan(/kernel-(.+).bc$/)[0][0]
         end
-        h
       end
+      h
+    end
+
+    def archs
+      @archs ||= default_archs
     end
 
     def arch_flags(platform)