ppc64_elf_edit_opd revamp

ppc64_elf_edit_opd revamp

This patch sorts .opd relocs (see pr17666) and allows .opd sections
with a mix of 16 and 24 byte entries to be edited.

	* elf64-ppc.c (OPD_NDX): Define.  Use throughout for sizing/indexing
	_opd_sec_data array, halving required memory.
	(sort_r_offset): New function.
	(ppc64_elf_edit_opd): Sort incoming relocs.  Accept .opd
	sections with a mix of 16 and 24 byte OPD entries.  Don't
	attempt to honour --non-overlapping-opd for .opd sections with
	unexpected relocs.  Simplify opd entry size calculations by
	first finding the reloc for the next entry.  Make edit loop
	handle one opd entry per iteration, with an inner loop
	handling relocs per entry.