Make powerpc use MAXARGS (defined as 8) instead of hardcoding '10'.

This brings its 'struct syscall_args' in sync with other architectures.

Reviewed by:	bdragon, jhibbits
MFC after:	2 weeks
Sponsored by:	EPSRC
Differential Revision:	https://reviews.freebsd.org/D26605

sys/powerpc/include/proc.h

@@ -59,10 +59,11 @@ struct mdproc {
 #define	KINFO_PROC_SIZE 816
 #endif
 
+#define	MAXARGS		8
 struct syscall_args {
 	u_int code;
 	struct sysent *callp;
-	register_t args[10];
+	register_t args[MAXARGS];
 };
 
 #ifdef _KERNEL