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_arm64.go

@@ -178,3 +178,15 @@ const (
 	SYS_EPOLL_CREATE = 1042
 	SYS_EPOLL_WAIT   = 1069
 )
+
+func Poll(fds []PollFd, timeout int) (n int, err error) {
+	var ts *Timespec
+	if timeout >= 0 {
+		ts = new(*Timespec)
+		*ts = NsecToTimespec(timeout * 1e6)
+	}
+	if len(fds) == 0 {
+		return ppoll(nil, 0, ts, nil)
+	}
+	return ppoll(&fds[0], len(fds), ts, nil)
+}