internal/task: swap stack chain when switching goroutines

This change swaps the stack chain when switching goroutines, ensuring that the chain is maintained consistently.
This is only really currently necessary with asyncify on wasm.

src/internal/task/task.go

@@ -15,6 +15,9 @@ type Task struct {
 	// Data is a field which can be used for storing state information.
 	Data uint64
 
+	// gcData holds data for the GC.
+	gcData gcData
+
 	// state is the underlying running state of the task.
 	state state
 }