libm: sync with upstream FreeBSD.

Test: ran tests
Change-Id: I16b5930b0dc652ceac60d8ed1d57c6126c74699c
diff --git a/libm/upstream-freebsd/lib/msun/src/s_cpow.c b/libm/upstream-freebsd/lib/msun/src/s_cpow.c
index 9be5c51..1cb99aa 100644
--- a/libm/upstream-freebsd/lib/msun/src/s_cpow.c
+++ b/libm/upstream-freebsd/lib/msun/src/s_cpow.c
@@ -44,11 +44,12 @@
  */
 
 #include <sys/cdefs.h>
-__FBSDID("$FreeBSD: head/lib/msun/src/s_cpow.c 336299 2018-07-15 00:23:10Z mmacy $");
+__FBSDID("$FreeBSD: head/lib/msun/src/s_cpow.c 336563 2018-07-20 18:27:30Z dim $");
 
 #include <complex.h>
 #include <float.h>
 #include <math.h>
+#include "math_private.h"
 
 double complex
 cpow(double complex a, double complex z)
@@ -60,7 +61,7 @@
 	y = cimag (z);
 	absa = cabs (a);
 	if (absa == 0.0) {
-		return (0.0 + 0.0 * I);
+		return (CMPLX(0.0, 0.0));
 	}
 	arga = carg (a);
 	r = pow (absa, x);
@@ -69,6 +70,6 @@
 		r = r * exp (-y * arga);
 		theta = theta + y * log (absa);
 	}
-	w = r * cos (theta) + (r * sin (theta)) * I;
+	w = CMPLX(r * cos (theta),  r * sin (theta));
 	return (w);
 }