Implement Math.log2 via ported extract from fdlibm.

src/third_party/fdlibm/fdlibm.cc

 // The following is adapted from fdlibm (http://www.netlib.org/fdlibm).
 //
 // ====================================================
 // Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 //
 // Developed at SunSoft, a Sun Microsystems, Inc. business.
 // Permission to use, copy, modify, and distribute this
 // software is freely granted, provided that this notice
 // is preserved.
 // ====================================================
@@ skipping 64 matching lines @@
     7.09782712893383973096e+02,   //           45  overflow threshold for expm1
     1.44269504088896338700e+00,   // 1/ln2     46
     -3.33333333333331316428e-02,  // Q1        47  coefficients for expm1
     1.58730158725481460165e-03,   //           48
     -7.93650757867487942473e-05,  //           49
     4.00821782732936239552e-06,   //           50
     -2.01099218183624371326e-07,  // Q5        51
     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
 };
 
 
 // Table of constants for 2/pi, 396 Hex digits (476 decimal) of 2/pi
 static const int two_over_pi[] = {
     0xA2F983, 0x6E4E44, 0x1529FC, 0x2757D1, 0xF534DD, 0xC0DB62, 0x95993C,
     0x439041, 0xFE5163, 0xABDEBB, 0xC561B7, 0x246E3A, 0x424DD2, 0xE00649,
     0x2EEA09, 0xD1921C, 0xFE1DEB, 0x1CB129, 0xA73EE8, 0x8235F5, 0x2EBB44,
     0x84E99C, 0x7026B4, 0x5F7E41, 0x3991D6, 0x398353, 0x39F49C, 0x845F8B,
     0xBDF928, 0x3B1FF8, 0x97FFDE, 0x05980F, 0xEF2F11, 0x8B5A0A, 0x6D1F6D,
@@ skipping 179 matching lines @@
   int n = __kernel_rem_pio2(tx, y, e0, nx);
   if (hx < 0) {
     y[0] = -y[0];
     y[1] = -y[1];
     return -n;
   }
   return n;
 }
 }
 }  // namespace v8::internal