Use cpack() and the gcc extension __imag__ to implement cimag() and

conj() instead of using expressions like z * I. The latter is bad for
several reasons:

1. It is implemented using arithmetic, which is unnecessary, and can
   generate floating point exceptions, contrary to the requirements on
   these functions.

2. gcc implements complex multiplication using a formula that breaks
   down for infinities, e.g., it gives INFINITY * I == nan + inf I.

lib/msun/src/s_cimag.c

@@ -31,5 +31,6 @@
 double
 cimag(double complex z)
 {
-	return -z * I;
+
+	return (__imag__ z);
 }

lib/msun/src/s_cimagf.c

@@ -31,5 +31,6 @@
 float
 cimagf(float complex z)
 {
-	return -z * I;
+
+	return (__imag__ z);
 }

lib/msun/src/s_cimagl.c

@@ -31,5 +31,6 @@
 long double
 cimagl(long double complex z)
 {
-	return -z * I;
+
+    return (__imag__ z);
 }

lib/msun/src/s_conj.c

@@ -28,8 +28,11 @@
 
 #include <complex.h>
 
+#include "math_private.h"
+
 double complex
 conj(double complex z)
 {
-	return creal(z) - I * cimag(z);
+
+	return (cpack(creal(z), -cimag(z)));
 }

lib/msun/src/s_conjf.c

@@ -28,8 +28,11 @@
 
 #include <complex.h>
 
+#include "math_private.h"
+
 float complex
 conjf(float complex z)
 {
-	return crealf(z) - I * cimagf(z);
+
+	return (cpackf(crealf(z), -cimagf(z)));
 }

lib/msun/src/s_conjl.c

@@ -28,8 +28,11 @@
 
 #include <complex.h>
 
+#include "math_private.h"
+
 long double complex
 conjl(long double complex z)
 {
-	return creall(z) - I * cimagl(z);
+
+	return (cpackl(creall(z), -cimagl(z)));
 }