misc/cgo/test: add mips64x test case for issue9400

Change-Id: If2b4abb6ff322c20e35de025298c8e5ab53edd42
Reviewed-on: https://go-review.googlesource.com/19808Reviewed-by: Minux Ma <minux@golang.org>

misc/cgo/test/issue9400/asm_mips64x.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 mips64 mips64le
+// +build !gccgo
+
+#include "textflag.h"
+
+#define SYNC	WORD $0xf
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT|NOFRAME,$0-0
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADDV	$(1024*8), R29
+
+	// Ask signaller to setgid
+	MOVW	$1, R1
+	SYNC
+	MOVW	R1, ·Baton(SB)
+	SYNC
+
+	// Wait for setgid completion
+loop:
+	SYNC
+	MOVW	·Baton(SB), R1
+	OR	R2, R2, R2	// hint that we're in a spin loop
+	BNE	R1, loop
+	SYNC
+
+	// Restore stack
+	ADDV	$(-1024*8), R29
+	RET