Add tests for all C11 headers.

Well, all except <threads.h> which we have no current plans to implement.

The only thing this turned up is that clang's <float.h> doesn't define
FLT_HAS_SUBNORM/DBL_HAS_SUBNORM/LDBL_HAS_SUBNORM, so for now we assert
that those are missing so that when we get a fixed clang, we notice.

Also the <tgmath.h> is utterly perfunctory, but seems like an appropriate
amount of effort to put in.

Bug: N/A
Test: ran tests
Change-Id: I7f7a7040b3271ceda625bee8a40a9d8cef52ee44
diff --git a/tests/stdalign_test.cpp b/tests/stdalign_test.cpp
new file mode 100644
index 0000000..fd6b81e
--- /dev/null
+++ b/tests/stdalign_test.cpp
@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <gtest/gtest.h>
+
+#include <stdalign.h>
+
+TEST(stdalign, smoke) {
+#if !defined(__alignas_is_defined) || __alignas_is_defined != 1
+#error __alignas_is_defined
+#endif
+#if !defined(__alignof_is_defined) || __alignof_is_defined != 1
+#error __alignof_is_defined
+#endif
+  ASSERT_EQ(1U, alignof(char));
+  struct alignas(128) S128 {};
+  ASSERT_EQ(128U, alignof(S128));
+}