test-user-mode: classify and skip all failing gem5 tests

Create userlan/posix/kill.c to better test signals.

userland/posix/kill.c

@@ -0,0 +1,16 @@
+/* A process that commits suicide by signal. */
+
+#define _XOPEN_SOURCE 700
+#include <signal.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int argc, char **argv) {
+    int sig;
+    if (argc <= 1) {
+        sig = 1;
+    } else {
+        sig = strtoull(argv[1], NULL, 0);
+    }
+    kill(getpid(), sig);
+}