Implement Math.log2 via ported extract from fdlibm.

src/third_party/fdlibm/fdlibm.cc

Index: src/third_party/fdlibm/fdlibm.cc
diff --git a/src/third_party/fdlibm/fdlibm.cc b/src/third_party/fdlibm/fdlibm.cc
index 23c13c0f058ba3091e9217de466650edb9153b00..0d014921bd646d88417c9708bd3f95d21e55a68b 100644
--- a/src/third_party/fdlibm/fdlibm.cc
+++ b/src/third_party/fdlibm/fdlibm.cc
@@ -82,7 +82,18 @@ const double MathConstants::constants[] = {
     710.4758600739439,            //           52  overflow threshold sinh, cosh
     4.34294481903251816668e-01,   // ivln10    53  coefficients for log10
     3.01029995663611771306e-01,   // log10_2hi 54
-    3.69423907715893078616e-13    // log10_2lo 55
+    3.69423907715893078616e-13,   // log10_2lo 55
+    5.99999999999994648725e-01,   // L1        56  coefficients for log2
+    4.28571428578550184252e-01,   //           57
+    3.33333329818377432918e-01,   //           58
+    2.72728123808534006489e-01,   //           59
+    2.30660745775561754067e-01,   //           60
+    2.06975017800338417784e-01,   // L6        61
+    9.61796693925975554329e-01,   // cp        62  2/(3*ln(2))
+    9.61796700954437255859e-01,   // cp_h      63
+    -7.02846165095275826516e-09,  // cp_l      64
+    5.84962487220764160156e-01,   // dp_h      65
+    1.35003920212974897128e-08    // dp_l      66
 };