Fix PR ld/20995 for cris-linux

	PR ld/20995
	* elf32-cris.c (elf_cris_size_dynamic_sections): Handle sdynrelro.
	(elf_cris_adjust_dynamic_symbol): Place variables copied into the
	executable from read-only sections into sdynrelro.
	(elf_cris_finish_dynamic_symbol): Select sreldynrelro for
	dynamic relocs in sdynrelro.
	(elf_backend_want_dynrelro): Define.