nohz_full: Add full-system idle states and variables

nohz_full: Add full-system idle states and variables

This commit adds control variables and states for full-system idle.
The system will progress through the states in numerical order when
the system is fully idle (other than the timekeeping CPU), and reset
down to the initial state if any non-timekeeping CPU goes non-idle.
The current state is kept in full_sysidle_state.

One flavor of RCU will be in charge of driving the state machine,
defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>