upload_system_symbols: Use the Go1.3 improvements to debug/macho.

This removes the custom MachO header reading functionality, since the stdlib can
now read Fat files.

R=andybons@chromium.org, mark@chromium.org

Review URL: https://breakpad.appspot.com/10684002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1360 4c0a9323-5329-0410-9bdc-e9ce6186880e