wfstack: implement mutex-free wfstack with transparent union

This allows users more freedom to use alternative synchronization
mechanisms.

Changes since v1:
- Fix typos in cds_wfs_stack_ptr_t documentation.
  Thanks to Mathieu for spotting.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>