wakeup wait for cpu display

Here's a slightly more constructive patch: this one adds support in
LTTV's generic state monitoring code and its control flow visualiser to
note when a process is woken by another process using the
sched_try_wakeup event.

Previously the woken process would continue to register its old state
until such time as it got scheduled; here the process transitions to
WAIT_CPU state (like preempted processes, indicating it is ready to run
but not currently scheduled). Ordinarily we see this state exist very
briefly, in between the device driver IRQ (typically) clearing it to run
and the scheduler invocation after irq_exit, but on a heavily loaded
system we might see a large stripe of dark yellow indicating the process
is ready but cannot yet be allocated a core.

The new code in controlflow/eventhooks.c is essentially a copy of the
second half of before_schedchange -- it would be nice to factor these
two and before_execmode, all of which basically identify a process,
create state objects if necessary, and draw his line up to a certain
time.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>