relocs_compatible test for gc-sections

relocs_compatible test for gc-sections

I noticed when looking at pr22300 that before calling check_relocs we
have an elf_object_id test (added for pr11933) as well as the
relocs_compatible test.  I believe backend gc_mark_hook and
gc_sweep_hook ought to be protected similarly from being confused by
unexpected relocations (for example, both elf64-ppc.c and elf32-ppc.c
use _bfd_elf_relocs_compatible, so I think it would be possible for
the ppc64 gc_mark_hook to be presented with a ppc32 relocatable
object).

	* elflink.c (elf_gc_sweep): Test elf_object_id in addition to
	relocs_compatible.
	(bfd_elf_gc_sections): Likewise.