sync: allow to work on armv5

asm_arm.s was using ldrex which does not work
on armv5. Tested on Sheevaplug.

R=rsc, kaib
CC=golang-dev
https://golang.org/cl/214049

src/pkg/sync/Makefile

@@ -9,7 +9,21 @@ GOFILES=\
 	mutex.go\
 	rwmutex.go\
 
+# 386-specific object files
+OFILES_386=\
+	asm_386.$O\
+
+# amd64-specific object files
+OFILES_amd64=\
+	asm_amd64.$O\
+
+GOARM?=6
+
+# arm-specific object files
+OFILES_arm=\
+	asm_arm$(GOARM).$O\
+
 OFILES=\
-	asm_$(GOARCH).$O\
+	$(OFILES_$(GOARCH))\
 
 include ../../Make.pkg

src/pkg/sync/asm_arm5.s

+// 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.
+
+// This version works on pre v6 architectures
+// func cas(val *int32, old, new int32) bool
+// Atomically:
+//	if *val == old {
+//		*val = new;
+//		return true;
+//	}else
+//		return false;
+
+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
+	MOVW	R0, 16(SP)	
+	RET
+fail1:	
+	MOVW	R3, (R4)	// release mutex
+fail0:
+	MOVW	$0, R0
+	MOVW	R0, 16(SP)	
+	RET
+