PR30824 internal error with -z pack-relative-relocs

This corrects a counting problem, where prior to relocate_section relr
encoded relative relocs were allowed when it was known they were on
even boundaries, but relocate_section can only put relative relocs
(non-relr) on eight byte boundaries.

	PR 30824
	* elf64-ppc.c (RELR_ALIGN): Define, use throughout.
	(maybe_relr): New function, use throughout.