Replace CR, FF and VT with whitespace.

We use Printlines to copy C code fragments to assembly comments.
While CR, FF and VT are treated like whitespace by C, they could
possibly mess up the assembly comments if copied verbatim.
Moreover, this avoids generating CR CR LF end-of-lines under Windows.
Bug 34075

lib/Printlines.ml

@@ -101,11 +101,13 @@ let copy oc pref b first last =
     try
       while b.lineno <= last do
         let c = input_char b.chan in
-        output_char oc c;
-        if c = '\n' then begin
+        match c with
+        | '\n' ->
+          output_char oc c;
           b.lineno <- b.lineno + 1;
           if b.lineno <= last then output_string oc pref
-        end
+        | '\r' | '\011' | '\012' -> output_char oc ' '
+        | _ -> output_char oc c
       done
     with End_of_file ->
       output_char oc '\n'