unix: fix Ppoll for arm64

Fixes golang/go#16065.

Change-Id: I97d06b7038e7c7507e765736899cfa243f45805c
Reviewed-on: https://go-review.googlesource.com/24121
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_arm64.go

@@ -182,8 +182,8 @@ const (
 func Poll(fds []PollFd, timeout int) (n int, err error) {
 	var ts *Timespec
 	if timeout >= 0 {
-		ts = new(*Timespec)
-		*ts = NsecToTimespec(timeout * 1e6)
+		ts = new(Timespec)
+		*ts = NsecToTimespec(int64(timeout) * 1e6)
 	}
 	if len(fds) == 0 {
 		return ppoll(nil, 0, ts, nil)