unix: extend z/OS support

extend support for z/OS target by adding the following syscalls:
- Sendfile (library implementation)
- Fcntl
- MmapPtr
- MunmapPtr

Change-Id: I098748802c2b275c15758e9d0132fca0490e9b70
GitHub-Last-Rev: 5aafd0b757c20e7c29d29e6286fd952ea494408f
GitHub-Pull-Request: golang/sys#223
Reviewed-on: https://go-review.googlesource.com/c/sys/+/620375
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>