RFC/A: Add a bfd hook for defining common symbols

[Dave Korn]

Nick Clifton wrote:
> Hi Richard,
> 
> 
>> OK, how does this look?  Tested in the same way as before.
> 
> Better - thanks.  You still have a mostly useless bfd * parameter, but I
> guess that there is a theoretical case where it might be needed.

  The rule of thumb I'd use in deciding whether to pass a bfd* as the first
arg to a new functions is "If I was writing this in C++, would it be an
ordinary member function of class BFD or a static one?"

    cheers,
      DaveK