Can I vent portable air conditioner into the garage?
The problem with venting the AC into the garage is that you are then venting the garage into your workout room.  I'd worry about CO poisoning.  Garages are typically air sealed from the rest of the house to prevent and CO from moving from the garage into the house.
I can't imagine a damper that I would trust enough to vent the AC into the garage and take the risk.  You could mitigate a bit by install a CO detector in your workout room but I'd still go with the drill to outside through stucco.
Drilling through stucco isn't such a huge deal.
The other thing to consider is that by venting the house into the garage which is presumably sealed from the house, you can depressurize your house (depending on your make up air vents).  A depressurized house can result in back drafting from combustion appliances.  In new construction there is typically a sheet metal expert who runs that calculations and ensures there is enough make up air available.  This factor would be similar whether you vent into the garage or outside though.
While you aren't using the garage for your car you may be storing things in the garage that off gas - paints, gas, etc.  If you depressurize the house and pressurize the garage there will be a propensity for the garage to want to leak into the house.