fix: allow building with userspace-rcu 0.13

fix: allow building with userspace-rcu 0.13

The liburcu detection code was checking for the 'synchronize_rcu_bp'
symbol in liburcu-bp which was dropped from the ABI in 0.13, add a
fallback check for 'urcu_bp_synchronize_rcu' which replaced it.

Also drop the check for 'call_rcu_bp' which we don't use and was just a
leftover of version check for 0.6 which has since been superseded.

Both those changes allow to build against liburcu >= 0.13.

Change-Id: I85c613fa0a41a4f78466501280768c4a4d4de12e
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>