rt: Reconfigure the C stack for valgrind each time it's used

src/rt/rust_task_thread.cpp

@@ -371,7 +371,6 @@ rust_task_thread::prepare_c_stack(rust_task *task) {
     I(this, !extra_c_stack);
     if (!cached_c_stack && !task->have_c_stack()) {
         cached_c_stack = create_stack(kernel, C_STACK_SIZE);
-        prepare_valgrind_stack(cached_c_stack);
     }
 }
 

src/rt/rust_task_thread.h

@@ -189,6 +189,7 @@ rust_task_thread::borrow_c_stack() {
         your_stack = cached_c_stack;
         cached_c_stack = NULL;
     }
+    prepare_valgrind_stack(your_stack);
     return your_stack;
 }