gdb: Use string_printf to format int fields instead of a fixed size buffer

This patch removes a FIXME comment from cli-out.c, now instead of
formatting integers into a fixed size buffer we build a std::string
and extract the formatted integer from that.

The old code using a fixed size buffer was probably fine (the integer
was not going to overflow it) and probably slightly more efficient
(avoids building a std::string) however, given we already have utility
code in GDB that will allow the 'FIXME' comment to be removed, it
seems like an easy improvement.

gdb/ChangeLog:

	* cli-out.c (cli_ui_out::do_field_int): Use string_printf rather
	than a fixed size buffer.

gdb/cli-out.c

@@ -94,14 +94,12 @@ void
 cli_ui_out::do_field_int (int fldno, int width, ui_align alignment,
 			  const char *fldname, int value)
 {
-  char buffer[20];	/* FIXME: how many chars long a %d can become? */
-
   if (m_suppress_output)
     return;
 
-  xsnprintf (buffer, sizeof (buffer), "%d", value);
+  std::string str = string_printf ("%d", value);
 
-  do_field_string (fldno, width, alignment, fldname, buffer);
+  do_field_string (fldno, width, alignment, fldname, str.c_str ());
 }
 
 /* used to omit a field */