Fix global_label_string_count for multiple arguments of bcgen

Setting label_module_n=-1 prevented the label name length to be counted towards the global_label_string_count when we would find a .export directive later on.

This happened for instance when calling bcgen on some module's ABC and _system.abc: the module refers to _cycle_in_spine and _print_graph, these are then found in _system.abc, but the string length was never counted, leading to a memory corruption in the bytecode parser.