Use atomic memory read/write functions in tests.

This ensures compiler optimisations won't interfere with the tests.

tests/test_helper.h

@@ -5,7 +5,7 @@
  * Provides some common definitions and helper functions for the memory
  * tests.
  *
- * Copyright (C) 2020 Martin Whitaker.
+ * Copyright (C) 2020-2021 Martin Whitaker.
  */
 
 #include <stddef.h>
@@ -13,6 +13,19 @@
 
 #include "test.h"
 
+/*
+ * Test word atomic read and write functions.
+ */
+#ifdef __x86_64__
+#include "memrw64.h"
+#define read_word   read64
+#define write_word  write64
+#else
+#include "memrw32.h"
+#define read_word   read32
+#define write_word  write32
+#endif
+
 /*
  * A wrapper for guiding branch prediction.
  */