Add initial support for .debug_sup sections.

Add initial support for .debug_sup sections.

	* dwarf.c (get_type_abbrev_from_form): Accept but ignore sup
	forms.
	(read_and_display_attr_value): Handle sup forms.
	(display_debug_sup): New function.  Displays the contents of a
	.debug_sup section.
	(load_debug_sup_file): New function.  Loads the contents of a file
	referenced by a .debug_sup section.
	(check_for_and_load_links): Call load_debug_sup_file.
	(debug_displays): Add entry for .debug_sup.
	* dwarf.h (enum dwarf_section_display_enum): Add debug_sup.
	* readelf.c (process_section_headers): Add support for debug_sup.
	* doc/debug.options.texi: Note that the =links option will display
	the contents of .debug_sup sections.
	* NEWS: Mention the new support.