sync: use mfence on windows-tcc (#6079)

vlib/sync/channel_2_test.v

@@ -0,0 +1,23 @@
+import sync
+
+const (
+	num_iterations = 10000
+)
+
+fn do_send(mut ch sync.Channel) {
+	for i in 0 .. num_iterations {
+		ch.push(&i)
+	}
+}
+
+fn test_channel_unbuffered() {
+	mut ch := sync.new_channel<int>(0)
+	go do_send(mut ch)
+	mut sum := i64(0)
+	for _ in 0 .. num_iterations {
+		a := 0
+		ch.pop(&a)
+		sum += a
+	}
+	assert sum == u64(num_iterations)*(num_iterations-1)/2
+}