Expand task_size variable for AArch32

Task size depends from VM_SPLIT_* kernel option and cannot be hard coded.
This patch based on c0c0546c from
Christopher Covington.

Signed-off-by: Artem Kuzmitskiy <artem.kuzmitskiy@lge.com>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>