The kern.rthreads sysctl has not existed for a long time - there is no way to
disable rthreads and __tfork no longer returns ENOTSUP.

Change-Id: Ia50ff01ac86ea83358e72b8f45f7818aaec1e4b1
Reviewed-on: https://go-review.googlesource.com/7490Reviewed-by: Minux Ma <minux@golang.org>