test-user-mode: make perfect like build-userland

Multithreading and target selection.

userland/c/infinite_loop.c

@@ -0,0 +1,29 @@
+/* Loop infinitely. Print an integer whenever a period is reached:
+ *
+ * ....
+ * ./infinite_loop [period]
+ * ....
+ */
+
+#include <inttypes.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv) {
+    uintmax_t i, j, period;
+    if (argc > 1) {
+        period = strtoumax(argv[1], NULL, 10);
+    } else {
+        period = 100000000;
+    }
+    i = 0;
+    j = 0;
+    while (1) {
+        i++;
+        if (i % period == 0) {
+            printf("%ju\n", j);
+            j++;
+        }
+    }
+}