* dwarf2.c: Add support for reading in debug information via a .gnu_debuglink section:

* dwarf2.c: Add support for reading in debug information via a .gnu_debuglink section:
  (struct dwarf2_debug): Add bfd field to record the bfd containing the debug info.
  (parse_comp_unit): Remove ABFD parameter.  Instead use the bfd field in the dwarf2_debug structure.
  (_bfd_dwarf2_find_nearest_line): If a debug info section could not be found in the current bfd call bfd_follow_gnu_debuglink to see if another file contains the debug information.  If it does, open it and continue.
  (bfd_dwarf2_find_line): Likewise.