GAS: DWARF-5: Ensure that the 0'th entry in the directory table contains the current working directory.

	* dwarf2dbg.c (get_directory_table_entry): Ensure that dir[0]
	contains current working directory.
	(out_dir_and_file_list): Likewise.
	* testsuite/gas/elf/dwarf-5-dir0.s: New test source file.
	* testsuite/gas/elf/dwarf-5-dir0.d: New test driver.
	* testsuite/gas/elf/elf.exp: Run the new test.
	* testsuite/gas/elf/dwarf-5-file0.d: Adjust expected output.
	* testsuite/gas/i386/dwarf5-line-1.d: Likewise.
	* testsuite/gas/i386/dwarf5-line-2.d: Likewise.