unix: expose mmap calls on z/OS

The calls `Mmap` and `Munmap` were removed for z/OS.
Syscall removed occurred in: 508397

This change exposes them in a z/OS specific file
(to remove the accidental deletion of them in future refactors of the APIs for `unix-like` OSes).

Change-Id: Ice0cabfb4547cab2ffa7130b7c26d9cc38233afe
GitHub-Last-Rev: e7b4680
GitHub-Pull-Request: #186
Reviewed-on: https://go-review.googlesource.com/c/sys/+/569359
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

unix/mmap_nomremap.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || darwin || dragonfly || freebsd || openbsd || solaris
+//go:build aix || darwin || dragonfly || freebsd || openbsd || solaris || zos
 
 package unix
 

unix/syscall_zos_s390x.go

@@ -1520,6 +1520,14 @@ func (m *mmapper) Munmap(data []byte) (err error) {
 	return nil
 }
 
+func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error) {
+        return mapper.Mmap(fd, offset, length, prot, flags)
+}
+
+func Munmap(b []byte) (err error) {
+        return mapper.Munmap(b)
+}
+
 func Read(fd int, p []byte) (n int, err error) {
 	n, err = read(fd, p)
 	if raceenabled {