Expose LLC's -force-align-stack flag to pnacl-translate

tests/toolchain/stackalign_test.c

+/*
+ * Copyright (c) 2014 The Native Client Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include <stdio.h>
+
+#include "native_client/src/include/nacl_assert.h"
+
+void call_with_misaligned_stack(void (*func)(void));
+
+volatile uintptr_t addrf;
+
+/*
+ * Test that locals are properly aligned, even if functions are called with a
+ * misaligned stack. This tests the effect of LLC's -force-align-stack (and also
+ * the fact that doubles must be 8-byte aligned). This test should only pass
+ * when using a flag that forces stack realignment.
+*/
+void testfunc(void) {
+  double f;
+  /*
+   * Smart compiler will optimize away the test (it assumes the alignment is
+   * correct) unless we write it to a volatile.
+   */
+  addrf = (uintptr_t) &f;
+  ASSERT_EQ(addrf % 8, 0);
+}
+
+int main(void) {
+  printf("Calling testfunc with properly aligned stack\n");
+  testfunc();
+  printf("Calling testfunc with misaligned stack\n");
+  call_with_misaligned_stack(testfunc);
+  return 0;
+}