libust events: lazy probe binding

libust events: lazy probe binding

Ability to cope with abritrary execution order of constructors, mainly
that libust constructor will always be called before the constructor of
the main programs (which will likely contain the probe callbacks and
descriptions from TRACEPOINT_EVENT) requires us to support lazy binding
between event and probes, "fixed" at probe registration time.

Allowing this lazy binding natively supports instrumentation of .so
(without changing the applications), and supports instrumenting shared
objects with dlopen().

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