lttng: probe callbacks

Implement the LTTng probe callbacks. One notable file here is
lttng-events.h, which is the core implementation of the LTTng
TRACE_EVENT macros for generation of probes and tracepoint decription
from the TRACE_EVENT declarations.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>