internal/task: add GoroutineStack stub for scheduler.none

The hardfault handler imports task.GoroutineStack(), which is only
defined for scheduler.tasks and scheduler.cores (Cortex-M C file).
With scheduler.none the function was missing, causing a build error.
With no scheduler, Thread mode runs on MSP and PSP is never set up,
so returning 0 is correct and causes the PSP block to be skipped.

Author: digitalentity

src/internal/task/task_none.go

@@ -42,3 +42,8 @@ func SystemStack() uintptr {
 	runtimePanic("scheduler is disabled")
 	return 0 // unreachable
 }
+
+func GoroutineStack() uintptr {
+	// No separate goroutine stack without a scheduler.
+	return 0
+}