Implement log10 via fdlibm port.

test/mjsunit/es6/math-log2-log10.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 21 matching lines @@
   assertTrue(isNaN(fun({ valueOf: function() { return -1; } })));
   assertTrue(isNaN(fun({ valueOf: function() { return "abc"; } })));
   assertTrue(isNaN(fun(-0.1)));
   assertTrue(isNaN(fun(-1)));
   assertEquals("-Infinity", String(fun(0)));
   assertEquals("-Infinity", String(fun(-0)));
   assertEquals(0, fun(1));
   assertEquals("Infinity", String(fun(Infinity)));
 });
 
-for (var i = -300; i < 300; i += 0.7) {
-  assertEqualsDelta(i, Math.log10(Math.pow(10, i)), 1E-13);
+for (var i = -310; i <= 308; i += 0.5) {
+  assertEquals(i, Math.log10(Math.pow(10, i)));
   assertEqualsDelta(i, Math.log2(Math.pow(2, i)), 1E-13);
 }
+
+// Test denormals.
+assertEquals(-307.77759430519706, Math.log10(1.5 * Math.pow(2, -1023)));