Fix for backslash interpretation in #line and #-markers (1/2)

[mike stump]

Just to be clear, I think this restores previous behavior and fixes a
bug introduced with the new preprocessor.  Further, routines that
output quoted strings into the assembly file must use something
consistent with:

  fprintf (asm_file, "\t.file\t");
  output_quoted_string (asm_file, na);
  fputc ('\n', asm_file);

(see toplev.c).  There was a systematic bug where people would just
dump the raw string, which is wrong.  I fixed all instances of it I
could find a while ago.  Sounds like either I missed a few, or some
crept back in.

Thanks Zack.