x/sys/unix: add support for zos

[billotosyr]

As discussed in golang-dev, I would like to add unix support for the s390x running zos. Other platforms should not be affected.

[gopherbot]

Change https://golang.org/cl/275992 mentions this issue: unix: add support for zos/s390x

[cagedmantis]

/cc @ianlancetaylor @bradfitz @tklauser