unix: update z/OS implementation of fcntl and mmap

- Add a wrapper function around fcntl to handle different operation
  types and new fcntl implementation that accepts uintptr as an arg.
- Add support for calling mmap/munmap with address pointers.
- Add accompanying tests for new functions.

Change-Id: If5e77aa4cf2cccfd431de4f3bd0c5014a761e167
GitHub-Last-Rev: 07e32a4ab797c7baffdb50f8407cf679cabf1dae
GitHub-Pull-Request: golang/sys#216
Reviewed-on: https://go-review.googlesource.com/c/sys/+/610296
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

unix/xattr_zos_test.go

@@ -14,7 +14,7 @@ import (
 )
 
 func TestXattr(t *testing.T) {
-	defer chtmpdir(t)()
+	chtmpdir(t)
 
 	f := "xattr1"
 	touch(t, f)