PR binutils/22875: HPPA/ELF: Also fail with relocation placeholders

Do not consider R_PARISC_UNIMPLEMENTED placeholder relocation entries of
the `elf_hppa_howto_table' table valid in `info_to_howto' HPPA handlers.
Instead issue an unsupported relocation type error and return a NULL
howto as with relocations whose number is R_PARISC_UNIMPLEMENTED or
beyond.

	bfd/
	* elf-hppa.h (elf_hppa_info_to_howto): Also return
	unsuccessfully for unimplemented relocations.
	(elf_hppa_info_to_howto_rel): Likewise.