msun: Rewrite function definitions with identifier lists

This syntax is removed in C2x proposal N2432.

Reviewed by:	pfg
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D35771

lib/msun/src/w_cabsf.c

@@ -15,8 +15,7 @@ static const char rcsid[] =
 #include "math_private.h"
 
 float
-cabsf(z)
-	float complex z;
+cabsf(float complex z)
 {
 
 	return hypotf(crealf(z), cimagf(z));

lib/msun/src/w_drem.c

@@ -8,8 +8,7 @@
 #include <math.h>
 
 double
-drem(x, y)
-	double x, y;
+drem(double x, double y)
 {
 	return remainder(x, y);
 }