elf: Add size_relative_relocs and finish_relative_relocs

On some targets, the DT_RELR section size can be computed only after all
symbols addresses can be determined.  Set the preliminary DT_RELR section
size before mapping sections to segments and set the final DT_RELR section
size after regular symbol processing is done.

	* elf-bfd.h (elf_backend_data): Add size_relative_relocs and
	finish_relative_relocs.
	* elf.c (_bfd_elf_map_sections_to_segments): Call
	size_relative_relocs if DT_RELR is enabled.
	* elflink.c (bfd_elf_final_link): Call finish_relative_relocs
	after regular symbol processing is finished if DT_RELR is enabled.
	* elfxx-target.h (elf_backend_size_relative_relocs): New.
	(elf_backend_finish_relative_relocs): Likewise.
	(elfNN_bed): Add elf_backend_size_relative_relocs and
	elf_backend_finish_relative_relocs.