Get lookups working (somehow) on ARM

Edit(Simon): This is a workaround for the register clash on ARM. r0
(ReturnValueRegister) is also used to pass the first argument, therefore we
can't use it as "temporary" register for the duration of the parameter setup.
Temporarily allocate r8 for now until we have constraints like these properly
propagated into the register allocator.

Change-Id: Iae266672856dea247b720367b8e8393333622e4e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>