gas/testsuite/ld-elf/dwarf2-21.d: Pass -W

gas/testsuite/ld-elf/dwarf2-21.d: Pass -W

Required for the expected "CU:" to be emitted for long
source-paths.  See binutils/dwarf.c:

 if (do_wide || strlen (directory) < 76)
   printf (_("CU: %s/%s:\n"), directory, file_table[0].name);
 else
   printf ("%s:\n", file_table[0].name);

See also commit 5f410aa50ce2c, "testsuite/ld-elf/pr26936.d:
Pass -W."

gas/ChangeLog:
	* testsuite/ld-elf/dwarf2-21.d: Pass -W.