rcutorture: Make scripts analyze rcuperf trace data, if present

The rcuperf event-trace data is more accurate than are the rcuperf
printk()s because locking keeps things ordered.  This commit therefore
parses and analyzes this event-trace data if present, and falls back on
the printk()s otherwise.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>