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.