Issue #12392: fix thread initialization on FreeBSD 6

On FreeBSD6, pthread_kill() doesn't work on the main thread before the creation
of the first thread. Create therefore a dummy thread (no-op) a startup to
initialize the pthread library.

Add also a test for this use case, test written by Charles-François Natali.
diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index 09a0887..fe9dde6 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -144,7 +144,10 @@
  * Initialization.
  */
 
-#ifdef _HAVE_BSDI
+/* On FreeBSD6, pthread_kill() doesn't work on the main thread before
+   the creation of the first thread */
+#if defined(_HAVE_BSDI) \
+    || (defined(__FreeBSD__) && __FreeBSD_version < 700000)
 static
 void _noop(void)
 {