Fix static linking: fix symbol name namespaces

gp_futex, yield_active, rand_yield, has_sys_membarrier, rcu_defer_exit,
call_rcu_data_free, call_rcu_before_fork, call_rcu_after_fork_parent,
call_rcu_after_fork_child are exported by each urcu flavor.

In order to fix use-cases where multiple flavors are statically linked
into the same application, we need to move these symbols to local
namespaces.

Ensure that all symbols are prefixed by "rcu_".

Also add each of those symbols into urcu/map/*.h headers, so they get
mapped to their flavor-specific symbol name by the preprocessor.

This requires bumping our .so version from 1.0.0 to 2.0.0, because it
changes some symbol names.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>