userland: move some multithreaded examples from cpp-cheat

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

userland/cpp/atomic.cpp

@@ -1,4 +1,4 @@
-// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+// https://github.com/cirosantilli/linux-kernel-module-cheat#cpp-multithreading
 //
 // The non-atomic counters have undefined values which get printed:
 // they are extremely likely to be less than the correct value due to

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++;
+    }
+}

userland/cpp/thread_hardware_concurrency.cpp

@@ -0,0 +1,10 @@
+// http://stackoverflow.com/questions/150355/programmatically-find-the-number-of-cores-on-a-machine
+//
+// Not affected by taskset: https://stackoverflow.com/questions/1006289/how-to-find-out-the-number-of-cpus-using-python/55423170#55423170
+
+#include <iostream>
+#include <thread>
+
+int main() {
+    std::cout << std::thread::hardware_concurrency() << std::endl;
+}