Implement Math.log1p using port from fdlibm.

third_party/fdlibm/fdlibm.cc

Index: third_party/fdlibm/fdlibm.cc
diff --git a/third_party/fdlibm/fdlibm.cc b/third_party/fdlibm/fdlibm.cc
index fe7e2316087b74ccdf3ed7d157176d5ade126c64..2f6eab17e8ac3fc9b9ccb6a14018803ec531500d 100644
--- a/third_party/fdlibm/fdlibm.cc
+++ b/third_party/fdlibm/fdlibm.cc
@@ -26,7 +26,7 @@ namespace fdlibm {
 inline double scalbn(double x, int y) { return _scalb(x, y); }
 #endif  // _MSC_VER
 
-const double TrigonometricConstants::constants[] = {
+const double MathConstants::constants[] = {
     6.36619772367581382433e-01,   // invpio2   0
     1.57079632673412561417e+00,   // pio2_1    1
     6.07710050650619224932e-11,   // pio2_1t   2
@@ -61,6 +61,17 @@ const double TrigonometricConstants::constants[] = {
     2.59073051863633712884e-05,   // T12      31
     7.85398163397448278999e-01,   // pio4     32
     3.06161699786838301793e-17,   // pio4lo   33
+    6.93147180369123816490e-01,   // ln2_hi   34
+    1.90821492927058770002e-10,   // ln2_lo   35
+    1.80143985094819840000e+16,   // 2^54     36
+    6.666666666666666666e-01,     // 2/3      37
+    6.666666666666735130e-01,     // LP1      38
+    3.999999999940941908e-01,     //          39
+    2.857142874366239149e-01,     //          40
+    2.222219843214978396e-01,     //          41
+    1.818357216161805012e-01,     //          42
+    1.531383769920937332e-01,     //          43
+    1.479819860511658591e-01,     // LP7      44
 };