Namespace lttng/align.h macros

These macros are part of the API and should have been namespaced
initialy. Namespace them and add backwards compatibility defines, make
sure to include the relevant system headers first for platforms that
have conflicting defines.

Change-Id: If2929ec2202dbde896614a25440ab200514ef4ad
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>