unix: add f_flag member flags on z/OS

This change adds the constants `ST_RDONLY` and `ST_NOSUID` to a z/OS-specific file.

Change-Id: Ifb78124d93b60a10f10b8169d1bb73dce074e760
GitHub-Last-Rev: 952cf1b
GitHub-Pull-Request: #214
Reviewed-on: https://go-review.googlesource.com/c/sys/+/606877
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: jlee3227

unix/zerrors_zos_s390x.go

@@ -581,6 +581,8 @@ const (
 	AT_EMPTY_PATH                   = 0x1000
 	AT_REMOVEDIR                    = 0x200
 	RENAME_NOREPLACE                = 1 << 0
+	ST_RDONLY                       = 1
+	ST_NOSUID                       = 2
 )
 
 const (