Linker support for m68k-elf from Andreas Schwab

Linker support for m68k-elf from Andreas Schwab
<schwab@issan.informatik.uni-dortmund.de>:

* elfcode.h (elf_link_add_object_symbols): Check for NULL when looping through
the symbol hashes.
(elf_bfd_final_link): When looking for _init and _fini don't use the symbol if
it is imported from another object.

* elf32-m68k.c: Relocation type changed from REL to RELA.  Regular and dynamic
linking support functions added, similar to other elf targets.