gas: Make sure .debug_line file table contains a zero filename and dir

gas: Make sure .debug_line file table contains a zero filename and dir

For DWARF5 the zero file list entry in the .debug_line table represents
the compile unit main file. It can be set with .file 0 when -gdwarf-5
is given. But since this directive is illegal for older versions, this
is almost never set. To make sure it is always set (so DW_AT_name of
the compile unit can be set) use file (and dir) 1 if that is defined
(otherwise fall back to pwd, to match DW_AT_comp_dir).

gas/ChangeLog:

	* gas/dwarf2dbg.c (out_dir_and_file_list): For DWARF5 emit at
	least one directory if there is at least one file. Use dirs[1]
	if dirs[0] is not set, or if there is no dirs[1] the current
	working directory. Use files[1] filename, when files[0] filename
	isn't set.