Back-end specific per-bfd data

[mark@codesourcery.com]

There seems to be no place to put back-end specific per-bfd data.
That means that some of the backends have resulted to static data,
which is surely not the right thing given the otherwise nicely
object-oriented and reentrant nature of bfd.

There is bfd->usrdata, but that is for *application* data, not
back-end specific data.

Am I missing the right hook for this?  Or, does anyone object to the
creation of bfd->used_by_bfd analagous to the field of the same name
in asection?

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com