Only use GNU cxxabi demangler for debug printing

Unfortunately, the cxxabi demanlger utility causes an error in
the serialization code. Until that is resolved (see also issue #811)
use the normal typeid string for serialization and cxxabi
for debug printing.

hpx/util/debug/demangle_helper.hpp

@@ -11,7 +11,7 @@
 #include <string>
 #include <type_traits>
 
-#if defined(__GNUC__)
+#if 0 && defined(__GNUC__)
 #include <cxxabi.h>
 #include <stdlib.h>