Minor changes to fix thread tracing

When threads are created before main (looking at you, CUDA), or are
not terminated before main terminates (looking at you, Iris), we need
to make sure that the registered new threads have a thread ID. To do
that, the top level timers for those threads should have synchronous
start/stop events, otherwise they get a single "unified" event at the
end with the thread ID belonging to the main thread (because it is
the one cleaning up a the end).

src/apex/apex.cpp

@@ -1925,6 +1925,7 @@ void register_thread(const std::string &name,
         std::shared_ptr<task_wrapper> twp =
             new_task(task_name, UINTMAX_MAX,
             (parent == nullptr ? task_wrapper::get_apex_main_wrapper() : parent));
+        twp->explicit_trace_start = true;
         start(twp);
         //printf("New thread: %p\n", &(*twp));
         thread_instance::set_top_level_timer(twp);

src/wrappers/pthread_wrapper_internal.cpp

@@ -108,6 +108,7 @@ class apex_wrapper {
     apex::register_thread("APEX pthread wrapper", parent);
     //_twp = apex::new_task((apex_function_address)func, UINTMAX_MAX, parent);
     _twp = apex::new_task((apex_function_address)func);
+    _twp->explicit_trace_start = true;
   }
   ~apex_wrapper() {
     this->stop();