Revert "Revert "Use compiler builtins for fabs.""

Don't enable the inlines when building libm itself. Otherwise clang gets
upset by seeing both an inline and a non-inline definition.

This reverts commit c5deb0f883.

Change-Id: If7abdb351f5a5549d6a331b33af408e8fcfa9868

libm/fabs.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2015 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 <math.h>
+
+#include "fpmath.h"
+
+double fabs(double x) {
+#if __arm__
+  // Both Clang and GCC insist on moving r0/r1 into a double register
+  // and using fabs where bit-twiddling would be a better choice.
+  // They get fabsf right, but we need to be careful in fabsl too.
+  IEEEd2bits u;
+  u.d = x;
+  u.bits.sign = 0;
+  return u.d;
+#else
+  return __builtin_fabs(x);
+#endif
+}
+
+float fabsf(float x) {
+  return __builtin_fabsf(x);
+}
+
+#if defined(__LP64__)
+long double fabsl(long double x) { return __builtin_fabsl(x); }
+#else
+long double fabsl(long double x) {
+  // Don't use __builtin_fabs here because of ARM. (See fabs above.)
+  return fabs(x);
+}
+#endif