catch urcu-paulmck.spin to my local version

The attached patch catches the repository up to my local version.
This adds the following:

o       Checking for no-progress cycles.

o       Allow enabling and disabling of update-side signal-mediated
        broadcast memory barriers via #define statements.

o       Enabling only those update-side signal-mediated broadcast
        memory barriers that are required for correctness.

This version assumes that synchronize_rcu() is locally ordered, for
example, with smp_mb() calls between each pair of major statements.
Please note that smp_mb() is -not- assumed within the loops nor
immediately before the second counter flip.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>