gcc47: Fix plugin mechanism

There were two problems with gcc 4.7 LTO plugin:
1) It needed a number of object files from libiberty.
   Unfortunately the previous object files couldn't be used as they need
   to be built with -fPIC and libiberty is only an internal (non-pic) lib.
2) The plugin header directory was missing completely, as seen with
   gcc -print-file-name=plugin only returning "plugin".