* elf64-mips.c (elf_mips_gnu_rel16_s2): Add internally used

	* elf64-mips.c (elf_mips_gnu_rel16_s2): Add internally used
	R_MIPS_GNU_REL16_S2 support.
	(bfd_elf64_bfd_reloc_type_lookup): Use it.
	(mips_elf64_rtype_to_howto): Use it.
	* elfn32-mips.c (elf_mips_gnu_rel16_s2): Add internally used
	R_MIPS_GNU_REL16_S2 support.
	(bfd_elf32_bfd_reloc_type_lookup): Use it.
	(mips_elf_n32_rtype_to_howto): Use it.