misc/cgo/test: add RewindAndSetgid implementation for s390x

Required to pass the issue9400 test.

Change-Id: I595223c403b12faade54e2e46510f8537150af39
Reviewed-on: https://go-review.googlesource.com/20940Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/issue9400/asm_s390x.s

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !gccgo
+
+#include "textflag.h"
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$0-0
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADD	$(1024 * 8), R15
+
+	// Ask signaller to setgid
+	MOVD	$·Baton(SB), R5
+	MOVW	$1, 0(R5)
+
+	// Wait for setgid completion
+loop:
+	SYNC
+	MOVW	·Baton(SB), R3
+	CMPBNE	R3, $0, loop
+
+	// Restore stack
+	SUB	$(1024 * 8), R15
+	RET