* elf32-mips.c, elf64-mips.c, elfn32-mips.c

	* elf32-mips.c, elf64-mips.c, elfn32-mips.c
	(elf_backend_merge_symbol_attribute): Define.
	* elflink.c (elf_link_add_object_symbols): Merge only the ELF
	visibility flags.
	* elfxx-mips.c (_bfd_mips_elf_merge_symbol_attribute): New function.