gas: Output directory and file names in .debug_line_str for DWARF5

gas: Output directory and file names in .debug_line_str for DWARF5

	* dwarf2dbg.c (add_line_strp): New function.
	(out_dir_and_file_list): Take line_seg and sizeof_offset as
	arguments, Use DW_FORM_line_strp for dir and file. Call
	add_line_strp and set symbol offset for DWARF2_LINE_VERSION 5.
	(out_debug_line): Call out_dir_and_file_list with line_seg and
	sizeof_offset.
	* gas/testsuite/gas/elf/dwarf-5-file0.d: Expect indirect line
	strings.