libitm: Fix race condition in dispatch choice at transaction begin.

	libitm/
	* beginend.cc (GTM::gtm_thread::begin_transaction): Move serial lock
	acquisition to ...
	* retry.cc (GTM::gtm_thread::decide_begin_dispatch): ... here.
	(default_dispatch): Make atomic.
	(GTM::gtm_thread::set_default_dispatch): Access atomically.
	(GTM::gtm_thread::decide_retry_strategy): Access atomically and
	use decide_begin_dispatch() if default_dispatch might have changed.
	(GTM::gtm_thread::number_of_threads_changed): Initialize
	default_dispatch here.

From-SVN: r184392