refperf: More closely synchronize reader start times

Currently, readers are awakened individually.  On most systems, this
results in significant wakeup delay from one reader to the next, which
can result in the first and last reader having sole access to the
synchronization primitive in question.  If that synchronization primitive
involves shared memory, those readers will rack up a huge number of
operations in a very short time, causing large perturbations in the
results.

This commit therefore has the readers busy-wait after being awakened,
and uses a new n_started variable to synchronize their start times.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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