Implement log10 via fdlibm port.

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

Index: test/mjsunit/es6/math-log2-log10.js
diff --git a/test/mjsunit/es6/math-log2-log10.js b/test/mjsunit/es6/math-log2-log10.js
index 4479894d7d89a0ebcd0ce4ecc6e8e5040e4f6421..4e69bb1641e2c01f3b37ac6123cc4332e0606b88 100644
--- a/test/mjsunit/es6/math-log2-log10.js
+++ b/test/mjsunit/es6/math-log2-log10.js
@@ -40,6 +40,12 @@
 });
 
 for (var i = -300; i < 300; i += 0.7) {
-  assertEqualsDelta(i, Math.log10(Math.pow(10, i)), 1E-13);
+  assertEqualsDelta(i, Math.log10(Math.pow(10, i)), 1E-16);

[Raymond Toy, 2014/11/19 19:02:30]

Nit: It would be useful to have a more accurate bound than 1e-16, in case log10
or pow changes slightly.

[Yang, 2014/11/20 09:05:10]

Done.

   assertEqualsDelta(i, Math.log2(Math.pow(2, i)), 1E-13);
 }
+
+for (var i = -311; i <= 308; i++) {
+  assertEquals(i, Math.log10(Math.pow(10, i)));
+}
+
+assertEquals(-307.77759430519706, Math.log10(1.5 * Math.pow(2, -1023)));

[Raymond Toy, 2014/11/19 19:02:30]

Add comment that you're testing a denormal case.

[Yang, 2014/11/20 09:05:10]

Done.