tidy elf_backend calls

tidy elf_backend calls

Function pointers in elfNN_bed that are initialized by elfxx-target.h
to non-zero values generally don't need a non-NULL test before calling
them.  Targets don't set a non-NULL function to NULL.  The one
exception being elfnn-ia64.c and that exception is removed here.

	* elf.c (_bfd_elf_setup_sections): Don't test known non-NULL
	backend functions for NULL before calling.
	(copy_special_section_fields, _bfd_elf_copy_private_bfd_data),
	(bfd_section_from_shdr, assign_section_numbers): Likewise.
	* elfcode.h (elf_write_relocs, elf_slurp_reloc_table): Likewise.
	* elfnn-ia64.c (ignore_errors): New function.
	(elf_backend_link_order_error_handler): Redefine as ignore_errors.