At least some versions of the Android libc do not define posix_openpt.

Updates #22845

Change-Id: Id21705f47ef0f9694313a7dc7351a952d48d407b
Reviewed-on: https://go-review.googlesource.com/79399
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>