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.

bfd/elfcode.h

@@ -987,12 +987,11 @@ elf_write_relocs (bfd *abfd, asection *sec, void *data)
       (*swap_out) (abfd, &src_rela, dst_rela);
     }
 
-  if (bed->write_secondary_relocs != NULL)
-    if (! bed->write_secondary_relocs (abfd, sec))
-      {
-	*failedp = TRUE;
-	return;
-      }
+  if (!bed->write_secondary_relocs (abfd, sec))
+    {
+      *failedp = TRUE;
+      return;
+    }
 }
 
 /* Write out the program headers.  */
@@ -1596,8 +1595,7 @@ elf_slurp_reloc_table (bfd *abfd,
 					      symbols, dynamic))
     return FALSE;
 
-  if (bed->slurp_secondary_relocs != NULL
-      && ! bed->slurp_secondary_relocs (abfd, asect, symbols))
+  if (!bed->slurp_secondary_relocs (abfd, asect, symbols))
     return FALSE;
 
   asect->relocation = relents;