Function multi_fork_child

tests/system_test/fork_test.cpp:102–106

std::atomic<int> g_multi_fork_done{0};

void multi_fork_child(void* arg) {
  g_multi_fork_done++;
  int code = static_cast<int>(reinterpret_cast<uintptr_t>(arg));
  sys_exit(code);
}

void test_fork_multiple(void* /*arg*/) {
  klog::Info("=== Fork Multiple Test ===");