gcov-dump: add --stable option

The option prints TOP N counters in a stable format
usage for comparison (diff).

gcc/ChangeLog:

	* doc/gcov-dump.texi: Document the new option.
	* gcov-dump.cc (main): Parse the new option.
	(print_usage): Show the option.
	(tag_counters): Sort key:value pairs of TOP N counter.