internal/poll: define PollDescriptor on plan9

Fixes #19114.

Change-Id: I352add53d6ee8bf78792564225099f8537ac6b46
Reviewed-on: https://go-review.googlesource.com/37106
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/internal/poll/fd_plan9.go

@@ -178,3 +178,7 @@ func isHangup(err error) bool {
 func isInterrupted(err error) bool {
 	return err != nil && stringsHasSuffix(err.Error(), "interrupted")
 }
+
+func PollDescriptor() uintptr {
+	return ^uintptr(0)
+}