cpu/native: fix thread_stack_init

The pointer arithmetic for the calculation of the context storage was off
due to the change of the stack's pointer type from unsigned int to char.
Fix offset calculation by not adjusting for unsigned int width anymore.

cpu/native/native_cpu.c

@@ -94,7 +94,7 @@ char *thread_stack_init(thread_task_func_t task_func, void *arg, void *stack_sta
 
     stk = stack_start;
 
-    p = (ucontext_t *)(stk + ((stacksize - sizeof(ucontext_t)) / sizeof(void *)));
+    p = (ucontext_t *)(stk + (stacksize - sizeof(ucontext_t)));
     stacksize -= sizeof(ucontext_t);
 
     if (getcontext(p) == -1) {