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

[Nick Clifton]

Hi Richard,

> What I'm trying to do is here is change something that ld/ does directly
> in such a way that some internal bfd datastructures are kept up to date.
> If we added a linker emulation hook, it would exist _purely_ for bookkeeping
> reasons, not semantic reasons.

I agree.  I also like your proposal, although I think that the new 
define-a-common function needs a bit of work.  Specifically:

   * Be paranoid - this is an API function - add some error checking of 
the inputs.  (eg H being NULL or H->type not being bfd_link_hash_common).

   * Add a boolean return value.  If the function fails to perform its 
duties it should set bfd_error and return FALSE.

   * The code in lang_one_common() took note of the setting of 
bfd_arch_mach_octets_per_byte (via the opb_shift variable), whereas you 
appear to have dropped this.

  * A more descriptive description would be nice.  Eg something like 
"Convert the common symbol H to a regular defined symbol.  Returns TRUE 
upon success, FALSE otherwise."


Cheers
   Nick