unix: add build tags to s390x files

Needed for old versions of the compiler.

Fixes golang/go#15697.

Change-Id: I913a4baef537775f037b736252b093a87e3b1015
Reviewed-on: https://go-review.googlesource.com/23130
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/asm_linux_s390x.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build s390x
+// +build linux
 // +build !gccgo
 
 #include "textflag.h"

unix/syscall_linux_s390x.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build s390x,linux
+
 package unix
 
 import (