[builtins] Introduce proper Float64Log1p operator.

src/base/ieee754.h

 // 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);
 
+// Returns a value equivalent to |log(1+x)|, but computed in a way that is
+// accurate even if the value of |x| is near zero.
+double log1p(double x);
+
 }  // namespace ieee754
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_IEEE754_H_