sync/atomic: disable store and load test on a single processor machine

Fixes #3226.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5756073

src/pkg/sync/atomic/atomic_test.go

@@ -1012,6 +1012,10 @@ func TestHammerStoreLoad(t *testing.T) {
 }
 
 func TestStoreLoadSeqCst32(t *testing.T) {
+	if runtime.NumCPU() == 1 {
+		t.Logf("Skipping test on %v processor machine", runtime.NumCPU())
+		return
+	}
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
 	N := int32(1e3)
 	if testing.Short() {
@@ -1049,6 +1053,10 @@ func TestStoreLoadSeqCst32(t *testing.T) {
 }
 
 func TestStoreLoadSeqCst64(t *testing.T) {
+	if runtime.NumCPU() == 1 {
+		t.Logf("Skipping test on %v processor machine", runtime.NumCPU())
+		return
+	}
 	if test64err != nil {
 		t.Logf("Skipping 64-bit tests: %v", test64err)
 		return
@@ -1090,6 +1098,10 @@ func TestStoreLoadSeqCst64(t *testing.T) {
 }
 
 func TestStoreLoadRelAcq32(t *testing.T) {
+	if runtime.NumCPU() == 1 {
+		t.Logf("Skipping test on %v processor machine", runtime.NumCPU())
+		return
+	}
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
 	N := int32(1e3)
 	if testing.Short() {
@@ -1132,6 +1144,10 @@ func TestStoreLoadRelAcq32(t *testing.T) {
 }
 
 func TestStoreLoadRelAcq64(t *testing.T) {
+	if runtime.NumCPU() == 1 {
+		t.Logf("Skipping test on %v processor machine", runtime.NumCPU())
+		return
+	}
 	if test64err != nil {
 		t.Logf("Skipping 64-bit tests: %v", test64err)
 		return