proc_mgmt: make sure that exit message is sent before things are cleaned up
proc_mgmt: make sure that exit message is sent before things are
cleaned up

This avoids getting stuck when a process is waiting for a process on the
same core to finish. In some cases the exit message was not sent by the
other process before the cleanup and it never reached the proc_mgmt

Signed-off-by: Roni Häcki <ac1438dbed87008d3b9d6c77a0ffddf380f86b06@inf.ethz.ch>

BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,kishoredbn/barrelfish