[Math builtins]: Cleanup in ieee754, restoring MSUN version of log2().

Also added comments for the fdlibm.js port of log10, chosen over the MSUN
version for greater precision.

R=bmeurer@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2071823002
Cr-Commit-Position: refs/heads/master@{#37060}

test/unittests/base/ieee754-unittest.cc

@@ -167,6 +167,7 @@ TEST(Ieee754, Log10) {
   EXPECT_EQ(3.7389561269540406, log10(5482.2158));
   EXPECT_EQ(14.661551142893833, log10(458723662312872.125782332587));
   EXPECT_EQ(-0.9083828622192334, log10(0.12348583358871));
+  EXPECT_EQ(5.0, log10(100000.0));
 }
 
 TEST(Ieee754, cbrt) {