rt: Add a task field to stk_seg and populate it

src/rt/rust_task.cpp

@@ -508,6 +508,7 @@ rust_task::new_stack(size_t requested_sz) {
     size_t sz = rust_stk_sz + RED_ZONE_SIZE;
     stk_seg *new_stk = create_stack(&local_region, sz);
     LOGPTR(thread, "new stk", (uintptr_t)new_stk);
+    new_stk->task = this;
     new_stk->next = NULL;
     new_stk->prev = stk;
     if (stk) {