zdtm/s390x_regs_check: Fix wait() for threads

For older kernels (e.g. RHEL7 with 3.10) it seems that wait(NULL) after
ptrace(PTHREAD_ATTACH) does not work properly for threads that have
to be created via clone().

Fix this by using waitpid() with the __WALL flag.

>From the waitpid() man page:

 __WALL (since Linux 2.4)
 Wait  for  all  children, regardless of type ("clone" or "non-clone").
Reported-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/s390x_regs_check.c

@@ -391,7 +391,10 @@ static void child_func(void)
 static int ptrace_attach(pid_t pid)
 {
 	if (ptrace(PTRACE_ATTACH, pid, 0, 0) == 0) {
-		wait(NULL);
+		if (waitpid(pid, NULL, __WALL) < 0) {
+			pr_perror("Waiting for thread %d failed", pid);
+			return -1;
+		}
 		return 0;
 	}
 	pr_perror("Attach to thread %d failed", pid);