PR 33743 readelf abort during RELR relocation handling

Rather than aborting, use "unknown relative" as the reloc type should
display_elf_relocation_at want to print a relocation.  Also fix
another case where readelf doesn't understand the relocation and sets
all_relocations[i].r_name to NULL.

	PR 33743
	* readelf.c (dump_relr_relocations): Don't abort on unknown
	e_machine.
	(dump_relocations): Don't assume we can pass NULL to a %s
	format.