sys.cas for mutex

R=rsc
APPROVED=rsc
DELTA=28  (28 added, 0 deleted, 0 changed)
OCL=31128
CL=31130

src/pkg/sync/asm_arm.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.
+
+// func cas(val *int32, old, new int32) bool
+// Atomically:
+//	if *val == old {
+//		*val = new;
+//		return true;
+//	}else
+//		return false;
+
+TEXT	sync·cas+0(SB),0,$12
+	TODO