runtime: disable TestParForParallel for now on 32-bit hosts

Also add call to GC() to make it easier to re-enable the test.

Update #4155.
When we have precise GC merged, re-enable this test.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6622058

src/pkg/runtime/parfor_test.go

@@ -92,6 +92,11 @@ func TestParForSetup(t *testing.T) {
 
 // Test parallel parallelfor.
 func TestParForParallel(t *testing.T) {
+	if GOARCH != "amd64" {
+		t.Log("temporarily disabled, see http://golang.org/issue/4155")
+		return
+	}
+
 	N := uint64(1e7)
 	if testing.Short() {
 		N /= 10
@@ -114,4 +119,7 @@ func TestParForParallel(t *testing.T) {
 			t.Fatalf("Wrong element %d: %d", i, data[i])
 		}
 	}
+
+	data, desc = nil, nil
+	GC()
 }