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_unix.go

@@ -0,0 +1,15 @@
+//go:build !windows && !linux
+// +build !windows,!linux
+
+/* 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) {
+	return unix.Poll(fds, timeout)
+}