Fix for coffcode.h

[DJ Delorie]

> I believe I found a typo in coffcode.h while working on revising the DJGPP 
> port. coff_write_object_contents() calls coff_swap_filehdr_out directly 
> instead of via the table.

Are you sure this is a bug?  coff_* may be a #define set according to
the file including coffcode.h, whereas bfd_coff_* may point somewhere
else entirely.  Or perhaps it's an optimization.  Is there a specific
case failing because of the way it is?