tools/memory-model: Add example for heuristic lockless reads

This commit adds example code for heuristic lockless reads, based loosely
on the sem_lock() and sem_unlock() functions.

[ paulmck: Apply Alan Stern and Manfred Spraul feedback. ]

Reported-by: Manfred Spraul <manfred@colorfullife.com>
[ paulmck: Update per Manfred Spraul and Hillf Danton feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>