Improved function proclaiming for GCL.

Quoting :doc note-7-0:

  (Should essentially affect GCL only) Significant re-work has been
  done for function proclaiming, which is still only done when the
  host Lisp is GCL. In particular, the build process now has extra
  steps even for other Lisps though for them, the extra steps are
  trivial (technical details may be found in the comment labeled
  ``Essay on Proclaiming'' in source file acl2-fns.lisp). We also
  have improved the calculation of return types. Thanks to Camm
  Maguire for helpful discussions, in particular for pointing out
  that the re-proclaiming that had been done previously could lead to
  buggy behavior.