* elf-bfd.h (struct elf_backend_data): New function pointer member

	elf_backend_merge_symbol_attribute.
	* elflink.h (elf_link_add_object_symbols): Adjust call to
	elf_backend_merge_symbol_attribute if the backend defined it.
        * elfxx-target.h (elf_backend_merge_symbol_attribute): New macro.
        (elfNN_bed): Add that to the initializer.