unix: add Ppoll support for linux

Use a shim for Poll on arm64.

Fixes golang/go#16052.

Change-Id: I929e7a2293561bddb9355bf65f98bc68b91905b2
Reviewed-on: https://go-review.googlesource.com/24062
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux_s390x.go

@@ -318,3 +318,12 @@ func Shutdown(s, how int) error {
 	}
 	return nil
 }
+
+//sys	poll(fds *PollFd, nfds int, timeout int) (n int, err error)
+
+func Poll(fds []PollFd, timeout int) (n int, err error) {
+	if len(fds) == 0 {
+		return poll(nil, 0, timeout)
+	}
+	return poll(&fds[0], len(fds), timeout)
+}