runtime: improve hardfault handler and stack reporting on Cortex-M

- Add reference to the current goroutine stack (PSP) when showing the hardfault info on Cortex-M.

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
+}

src/internal/task/task_stack_cortexm.c

@@ -11,3 +11,14 @@ uintptr_t SystemStack() {
     );
     return sp;
 }
+
+uintptr_t GoroutineStack() {
+    uintptr_t sp;
+    asm volatile(
+        "mrs %0, PSP"
+        : "=r"(sp)
+        :
+        : "memory"
+    );
+    return sp;
+}

src/internal/task/task_stack_cortexm.go

@@ -70,3 +70,10 @@ func (s *state) pause() {
 //
 //export SystemStack
 func SystemStack() uintptr
+
+// GoroutineStack returns the PSP (goroutine stack pointer). When a fault
+// occurs in goroutine context, the hardware saves the exception frame to PSP;
+// reading PSP+0x18 gives the actual faulting PC.
+//
+//export GoroutineStack
+func GoroutineStack() uintptr