Restore readelf's warnings that describe real problems with the file being examined...

Restore readelf's warnings that describe real problems with the file being examined.  Fix bug displaying empty file name tables.

binutils* dwarf.c (do_checks): New global variable.
	(display_formatted_table): Warn about an unexpected number of
	columns in the table, if checks are enabled.  Do not complain
	about the lack of data following the number of entries in the
	table if the table is empty.
	(display_debug_lines_decoded): Only warn about an unexpected
	number of columns in a table if checks are enabled.
	* dwarf.h (do_checks): Add a prototype.
	* elfcomm.c (error): Remove weak attribute.
	(warn): Likewise.
	* readelf.c (do_checks): Delete.
	(warn): Delete.
	(process_section_headers): Only warn about empty sections if
	checks are enabled.

gas	* dwarf2dbg.c (out_dir_and_file_list): Add comments describing the
	construction of a DWARF-5 directory name table.
	* testsuite/gas/elf/pr25917.d: Update expected output.