userland: move some multithreaded examples from cpp-cheat

Using them mostly to evaluate how well the emulators are handling user
mode multithreading.

userland/cpp/count.cpp

@@ -0,0 +1,16 @@
+// Count to infinity sleeping one second per number.
+//
+// https://github.com/cirosantilli/linux-kernel-module-cheat#cpp-multithreading
+
+#include <chrono>
+#include <thread>
+#include <iostream>
+
+int main() {
+    int i = 0;
+    while (1) {
+        std::cout << i << std::endl;
+        std::this_thread::sleep_for(std::chrono::seconds(1));
+        i++;
+    }
+}