No.
The initial response will be a loss of lift due to the loss of dynamic pressure, associated with the drop in airspeed. The airflow will remain attached, therefore it's not a stall.
What happens next MAY be a stall, but it all depends what the pilot (or aircraft) does. If he pulls back to 'regain the lift' he may well stall. If he bunts (perhaps non-intuitive) to try to regain speed the wing will unload further and the plane will not stall. (Of course, it may instead hit the ground; that's the nature of windshear)