testsuite/ld-elf/pr26936.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);