Move thread object mark into thread root mark.

No reason for it to specially be mentioned in collect.c.

src/gc/collect.c

@@ -78,10 +78,6 @@ void MVM_gc_collect(MVMThreadContext *tc, MVMuint8 what_to_do, MVMuint8 gen) {
         tc->nursery_alloc       = tospace;
         tc->nursery_alloc_limit = (char *)tc->nursery_alloc + MVM_NURSERY_SIZE;
 
-        MVM_gc_worklist_add(tc, worklist, &tc->thread_obj);
-        GCDEBUG_LOG(tc, MVM_GC_DEBUG_COLLECT, "Thread %d run %d : processing %d items from thread_obj\n", worklist->items);
-        process_worklist(tc, worklist, &wtp, gen);
-
         /* Add permanent roots and process them; only one thread will do
         * this, since they are instance-wide. */
         if (what_to_do != MVMGCWhatToDo_NoInstance) {

src/gc/roots.c

@@ -88,6 +88,9 @@ void MVM_gc_root_add_tc_roots_to_worklist(MVMThreadContext *tc, MVMGCWorklist *w
         cur_ah = cur_ah->next_handler;
     }
 
+    /* The thread object. */
+    MVM_gc_worklist_add(tc, worklist, &tc->thread_obj);
+
     /* Any exception handler result. */
     MVM_gc_worklist_add(tc, worklist, &tc->last_handler_result);