misc/cgo/test: add mipsx test case for #9400

Change-Id: I7d0bc5093943b0744d865e91517ff6292f3b2f89
Reviewed-on: https://go-review.googlesource.com/34510
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

misc/cgo/test/issue9400/asm_mipsx.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 mips mipsle
+// +build !gccgo
+
+#include "textflag.h"
+
+TEXT ·RewindAndSetgid(SB),NOSPLIT,$-4-0
+	// Rewind stack pointer so anything that happens on the stack
+	// will clobber the test pattern created by the caller
+	ADDU	$(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
+	ADDU	$(-1024*8), R29
+	RET