lib/libm_dbl/tanh: Make tanh more efficient and handle large numbers.

Prior to this patch tanh(large number) would return nan due to inf/inf.

lib/libm_dbl/tanh.c

@@ -1,5 +1,12 @@
 #include <math.h>
 
 double tanh(double x) {
-    return sinh(x) / cosh(x);
+    int sign = 0;
+    if (x < 0) {
+        sign = 1;
+        x = -x;
+    }
+    x = expm1(-2 * x);
+    x = x / (x + 2);
+    return sign ? x : -x;
 }