Add ability to follow dwo links to readelf/objdump.

Add ability to follow dwo links to readelf/objdump.

	* dwarf.c (dwo_name, dwo_dir, dwo_id, dwo_id_len): New variables.
	(read_and_display_attr_value): Record dwo variables if requested.
	(display_augmentation_data): Rename to display_data and make
	generic.
	(load_dwo_file): New function.  Loads a separate dwarf object
	file.
	(load_separate_debug_file): Add reporting and loading of separate
	dwarf objet files.
	* readelf.c (process_section_headers): Add do_debug_links to list
	of flags requiring a debug dump.
	(display_debug_section): Tidy up code.
	* doc/debug.options.texi: Add note that dwo links will also be
	followed.
	* testsuite/binutils-all/debuglink.s: Tidy code.
	* testsuite/binutils-all/dwo.s: New test file.
	* testsuite/binutils-all/readelf.wk2: New file - expected output
	from readelf.
	* testsuite/binutils-all/readelf.exp: Run the new test.