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

vlib/sync/channel_4_test.v

@@ -0,0 +1,38 @@
+import sync
+
+fn do_rec(mut ch sync.Channel, mut resch sync.Channel) {
+	mut sum := i64(0)
+	for _ in 0 .. 2000 {
+		mut a := 0
+		ch.pop(&a)
+		sum += a
+	}
+	println(sum)
+	resch.push(&sum)
+}
+
+fn do_send(mut ch sync.Channel) {
+	for i in 0 .. 2000 {
+		ch.push(&i)
+	}
+}
+
+fn test_channel_multi_buffered() {
+	mut ch := sync.new_channel<int>(100)
+	mut resch := sync.new_channel<i64>(0)
+	go do_rec(mut ch, mut resch)
+	go do_rec(mut ch, mut resch)
+	go do_rec(mut ch, mut resch)
+	go do_rec(mut ch, mut resch)
+	go do_send(mut ch)
+	go do_send(mut ch)
+	go do_send(mut ch)
+	go do_send(mut ch)
+	mut sum := i64(0)
+	for _ in 0 .. 4 {
+		mut r := i64(0)
+		resch.pop(&r)
+		sum += r
+	}
+	assert sum == i64(4) * 2000 * (2000 - 1) / 2
+}