[builtins] Introduce proper base::ieee754::log.

src/base/ieee754.h

Index: src/base/ieee754.h
diff --git a/src/base/ieee754.h b/src/base/ieee754.h
new file mode 100644
index 0000000000000000000000000000000000000000..3622320b46964bdf611a477afd39e4e709432d00
--- /dev/null
+++ b/src/base/ieee754.h
@@ -0,0 +1,19 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_BASE_IEEE754_H_
+#define V8_BASE_IEEE754_H_
+
+namespace v8 {
+namespace base {
+namespace ieee754 {
+
+// Returns the natural logarithm of |x|.
+double log(double x);
+
+}  // namespace ieee754
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_IEEE754_H_