Switch from pthread hijacking back to portable native sync for gtk AWT threading

Patch 12651 (reverted by patch 12666) reintroduced portable native sync for the gtk peer AWT threading. It removed a syswrap hijack of 2 pthread calls which prevented deadlock in GTk code. This introduced an inefficiency in the way scheduled AWT threads, but this was better than a deadlock. We should re-apply 12651 but we can only do this if Classpath is rebuilt on the test infastructure machines with --enable-portable-native-sync on their configure scripts. More generally the test infrastructure machines should rebuild Classpath.