Implement gdbsrv "v.info stats" command giving statistics for valgrind core + tools

* addition of VG_(needs_print_stats) in pub_tool_tooliface.h
* use the above in memcheck and helgrind
* output valgrind stats and calls print_stats in server.c



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13760 a5019735-40e9-0310-863c-91ae7b9d1cf9