cpp: parametrize nthreads on threaded programs CLI

Mark them as multithreaded in path_properties. Preparation for
https://cirosantilli.com/linux-kernel-module-cheat/#user-mode-static-executables-with-dynamic-libraries
which is likely what breaks gem5 on them.

userland/cpp/thread_get_id.cpp

@@ -4,6 +4,7 @@
 //
 // On Ubuntu 19.04, they ar large possibly non-consecutive numbers.
 
+#include <cstring>
 #include <iostream>
 #include <mutex>
 #include <thread>
@@ -12,15 +13,30 @@
 std::mutex mutex;
 
 void myfunc(int i) {
+    // Mutex and flush to prevent the output from
+    // different threads from interleaving.
     mutex.lock();
-    std::cout << i << " " << std::this_thread::get_id() << std::endl;
+    std::cout <<
+        i << " " <<
+        std::this_thread::get_id() << std::endl
+        << std::flush;
     mutex.unlock();
 }
 
-int main() {
+int main(int argc, char **argv) {
     std::cout << "main " << std::this_thread::get_id() << std::endl;
     std::vector<std::thread> threads;
-    for (unsigned int i = 0; i < 4; ++i) {
+    unsigned int nthreads;
+
+    // CLI arguments.
+    if (argc > 1) {
+        nthreads = std::strtoll(argv[1], NULL, 0);
+    } else {
+        nthreads = 1;
+    }
+
+    // Action.
+    for (unsigned int i = 0; i < nthreads; ++i) {
         threads.push_back(std::thread(myfunc, i));
     }
     for (auto& thread : threads) {