Round down the location of execpathp to slightly improve copyout speed.

In practice, this moves the padding from below the canary to above
execpathp has no impact on stack consumption.

Submitted by:		Wuyang-Chung (via github pull request #159)
MFC after:	1 week

sys/kern/kern_exec.c

@@ -1512,6 +1512,7 @@ exec_copyout_strings(struct image_params *imgp)
 	 */
 	if (execpath_len != 0) {
 		destp -= execpath_len;
+		destp = rounddown2(destp, sizeof(void *));
 		imgp->execpathp = destp;
 		copyout(imgp->execpath, (void *)destp, execpath_len);
 	}