dtrace: support passing offset as arg0 to FBT return probes

dtrace: support passing offset as arg0 to FBT return probes

FBT return probes pass the offset from the function start (in bytes)
as arg0.  To make that possible, we pass the offset value in the call
to fbt_add_probe.  For FBT entry probes we pass 0 (which is ignored).

This commit also ensures that we emulate the 'ret' instruction on the
return path.

Orabug: 25949086
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Tomas Jedlicka <tomas.jedlicka@oracle.com>
Acked-by: Nick Alcock <nick.alcock@oracle.com>