runtime: Make exit() power off the VM

Use osv::poweroff() instead of abort() in exit() to make the VM stop
cleanly on JVM System.exit().

Needed by DaCapo.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
benchmark dacapo

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>