On ix86, add -Wno-format-overflow to --enable-gdb-build-warnings.
    
      (This is a workaround for the bogus warning/error that we now see
      on i686 regarding a "may write a terminating nul past the end of
      the destination" message for the sprintf() call in
      global_symbol_searcher::search() in gdb/symtab.c.)