score howto special_function final linking output bfd access

Like commit 804439b4d7a8, avoid segfaults when the hack to access
the output bfd won't work, eg. when the sym is absolute.

	* elf32-score.c (score_elf_gprel15_reloc): Return bfd_reloc_undefined
	when output_bfd cannot be determined from the symbol.
	(score_elf_gprel32_reloc): Likewise.
	* elf32-score7.c (score_elf_gprel32_reloc): Likewise.