Move DT_RELR tag setting to elflink.c

This makes the code setting DT_RELR tags generally available.  Many
targets will be able to use the defaults.  Those that can't should set
up sh_entsize for .relr.dyn output section before reaching the dynamic
tag code in bfd_elf_final_link.

	* elflink.c (bfd_elf_final_link): Set up DT_RELR tags and sh_entsize.
	* elfxx-x86.c (_bfd_x86_elf_finish_dynamic_sections): Don't do any
	of that here.