Update task_stack_cortexm.c

Added build tag.

src/internal/task/task_stack_cortexm.c

@@ -1,3 +1,4 @@
+//go:build scheduler.tasks && cortexm
 #include <stdint.h>
 
 uintptr_t SystemStack() {
@@ -9,4 +10,4 @@ uintptr_t SystemStack() {
         : "memory"
     );
     return sp;
-}
+}