objdump: don't cache section contents in load_specific_debug_section

	* objdump.c (load_specific_debug_section): Don't call
	bfd_cache_section_contents.  Rearrange so that
	bfd_get_full_section_contents is not called on path where
	bfd_simple_get_relocated_section_contents is called.
	Don't set section->user_data.
	(free_debug_section): Always free section->start.  Don't twiddle
	section flags.
	* readelf.c (load_specific_debug_section): Don't set user_data.
	* dwarf.h (struct dwarf_section): Remove use_data field.
	* dwarf.c (NO_ABBREVS, ABBREV): Adjust to suit.