libthreads: pass 'class Thread' object as ModelAction 'location'

libthreads: pass 'class Thread' object as ModelAction 'location'

Currently, the 'location' parameter is unused for THREAD_CREATE. I need to
provide a reference from a Thread object to the ModelAction which created it,
so I will use this parameter as an entry point into the ModelChecker, which can
build the necessary reference later.