At runtime, UST checks if the LTTng clock source is available

At runtime, UST checks if the LTTng clock source is available
If not, CLOCK_MONOTONIC is the default fallback.

This makes UST use the LTTng kernel clock source. With this
clock support, LTTng and UST traces are perfectly synchronize.
Traces from these two tracer can be merged for analysis.

LTTng 0.240 or higher is needed for this new clock source.

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: David Goulet <david.goulet@polymtl.ca>
Acked-by: Nils Carlson <nils.carlson@ericsson.com>