Add debug facility for formatting in rr sessions

The existing debug print commands don't work in rr, since they touch
stderr. The new xfmt command just calls Fformat and doesn't touch the
stdio streams.

* src/.gdbinit:
(xfmt): New GDB command.

* src/print.c:
(debug_format): New function.

src/.gdbinit

@@ -119,6 +119,12 @@ Print the value of the lisp variable given as argument.
 Works only when an inferior emacs is executing.
 end
 
+# Format the value and print it as a string. Works in
+# an rr session and during live debugging. Calls into lisp.
+define xfmt
+  printf "%s\n", debug_format("%S", $arg0)
+end
+
 # Print out current buffer point and boundaries
 define ppt
   set $b = current_buffer

src/print.c

@@ -855,6 +855,17 @@ safe_debug_print (Lisp_Object arg)
     }
 }
 
+/* This function formats the given object and returns the result as a
+   string. Use this in contexts where you can inspect strings, but
+   where stderr output won't work --- e.g., while replaying rr
+   recordings.  */
+const char * debug_format (const char *, Lisp_Object) EXTERNALLY_VISIBLE;
+const char *
+debug_format (const char *fmt, Lisp_Object arg)
+{
+  return SSDATA (CALLN (Fformat, build_string (fmt), arg));
+}
+
 
 DEFUN ("error-message-string", Ferror_message_string, Serror_message_string,
        1, 1, 0,