Libdw: Remove special treatment for stg_stop_thread

This is no longer necessary since this symbol can be unwound through
with its DWARF information.

rts/Libdw.c

@@ -230,10 +230,7 @@ static int frame_cb(Dwfl_Frame *frame, void *arg) {
         backtrace_push(session->cur_bt, (StgPtr) pc);
     }
 
-    if ((void *) pc == &stg_stop_thread_info)
-        return DWARF_CB_ABORT;
-    else
-        return DWARF_CB_OK;
+    return DWARF_CB_OK;
 }
 
 Backtrace *libdw_get_backtrace(LibDwSession *session) {