V4 JIT: fix LookupCall on ARM

V4 JIT: fix LookupCall on ARM

To generate a LookupCall, the register r8 was used on ARM instead of the
ReturnValue register. The reason is that the ReturnValue register is
also the register for the first argument. However, now that we use
callee-saved registers (r8 among them), this would clobber any value
stored in r8. The fix is to actually use r0 to calculate the value,
because the first argument holds the lookup table, and the call is
relative to that. This leaves r8 free to be used by the register
allocator.

Change-Id: I5095bf69d27e16111ad32d9e5d5691c7bce14516
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>