* elf-bfd.h (struct elf_backend_data): Add gc_mark_dynamic_ref.

	* elf-bfd.h (struct elf_backend_data): Add gc_mark_dynamic_ref.
	(bfd_elf_gc_mark_dynamic_ref_symbol): Declare.
	* elflink.c (bfd_elf_gc_mark_dynamic_ref_symbol): Rename from
	elf_gc_mark_dynamic_ref_symbol.  Make global.
	(bfd_elf_gc_sections): Call bed->gc_mark_dynamic_ref.
	* elfxx-target.h (elf_backend_gc_mark_dynamic_ref): Define.
	(elfNN_bed): Init new field.
	* elf64-ppc.c (elf_backend_gc_mark_dynamic_ref): Define.
	(ppc64_elf_gc_mark_dynamic_ref): New function.