rwcancel: use ppoll on Linux for Android

This is a temporary measure while we wait for
https://go-review.googlesource.com/c/sys/+/352310 to land.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

rwcancel/poll_linux.go

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package rwcancel
+
+import "golang.org/x/sys/unix"
+
+func poll(fds []unix.PollFd, timeout int) (n int, err error) {
+	var ts *unix.Timespec
+	if timeout >= 0 {
+		ts = new(unix.Timespec)
+		*ts = unix.NsecToTimespec(int64(timeout) * 1e6)
+	}
+	return unix.Ppoll(fds, ts, nil)
+}