add support for pre arm v6 cas. set GOARM=5 to enable.

R=rsc
https://golang.org/cl/154101

src/pkg/runtime/Makefile

@@ -28,9 +28,11 @@ OFILES_386=\
 	vlop.$O\
 	vlrt.$O\
 
+GOARM ?= 6
+
 # arm-specific object files
 OFILES_arm=\
-	cas.$O\
+	cas$(GOARM).$O\
 	memset.$O\
 	vlop.$O\
 	vlrt.$O\

src/pkg/runtime/arm/cas5.s

@@ -0,0 +1,43 @@
+// Copyright 2009 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.
+
+#include "arm/asm.h"
+
+// This version works on pre v6 architectures
+
+// bool cas(int32 *val, int32 old, int32 new)
+// Atomically:
+//	if(*val == old){
+//		*val = new;
+//		return 1;
+//	}else
+//		return 0;
+
+TEXT	cas(SB),7,$0
+	MOVW	0(FP), R0	// *val
+	MOVW	4(FP), R1	// old
+	MOVW	8(FP), R2	// new
+	MOVW	$1, R3
+	MOVW	$cas_mutex(SB), R4
+l:
+	SWPW	(R4), R3	// acquire mutex
+	CMP		$0, R3
+	BNE		fail0
+	
+	MOVW	(R0), R5
+	CMP		R1, R5
+	BNE		fail1
+	
+	MOVW	R2, (R0)	
+	MOVW	R3, (R4)	// release mutex
+	MOVW	$1, R0
+	RET
+fail1:	
+	MOVW	R3, (R4)	// release mutex
+fail0:
+	MOVW	$0, R0
+	RET
+ 
+DATA cas_mutex(SB)/4, $0
+GLOBL cas_mutex(SB), $4