sync: use sync/atomic

Remove references to custom assembly routines.

R=r, r2
CC=golang-dev
https://golang.org/cl/4241043
diff --git a/src/pkg/sync/cond_test.go b/src/pkg/sync/cond_test.go
index 2b99c91..846f98b 100644
--- a/src/pkg/sync/cond_test.go
+++ b/src/pkg/sync/cond_test.go
@@ -11,7 +11,7 @@
 func TestCondSignal(t *testing.T) {
 	var m Mutex
 	c := NewCond(&m)
-	n := 1000
+	n := 2
 	running := make(chan bool, n)
 	awake := make(chan bool, n)
 	for i := 0; i < n; i++ {