GAS: Do not create an entry for the default directory if the directory table is empty...

GAS: Do not create an entry for the default directory if the directory table is empty.  Improve readelf's decoding of empty directory and file name tables.

	PR 25917
	* dwarf.c (display_debug_lines_decoded): Warn if encountering a
	supicious number of entries for DWARF-5 format directory and file
	name tables.  Do not display file name table header if the table
	is empty.  Do not allocate space for empty tables.