qsort: elf_sort_sections use of target_index

qsort: elf_sort_sections use of target_index

elf_sort_sections tried to ensure a stable qsort by using target_index
as the final comparison, but target_index hasn't been set by anything
at the time elf_sort_sections was run.  This patch arrange to have
target_index set.

	* elf.c (_bfd_elf_map_sections_to_segments): Init target_index
	for sections about to be sorted.
	(assign_file_positions_for_load_sections): Likewise.
	(elf_sort_sections): Don't bother optimising both TOEND case.
	* elflink.c (bfd_elf_final_link): Reset target_index.