Do not consider placeholder EMPTY_HOWTO relocation entries valid in
`rtype_to_howto' MIPS handlers.  Instead issue an unsupported relocation
type error and return a NULL howto as with relocations outside the three
ISA-specific min-max ranges.

	bfd/
	* elf32-mips.c (mips_elf32_rtype_to_howto): Also return
	unsuccessfully for placeholder howtos.
	* elf64-mips.c (mips_elf64_rtype_to_howto): Likewise.
	* elfn32-mips.c (mips_elf_n32_rtype_to_howto): Likewise.