Fix build on ARM in ARM mode (instead of thumb2)

We don't support the traditional ARM assembler (yet), only JIT on thumb2.  In
order for us to reliably check that, we have to wait until the pre-processor
runs, which this patch achieves by moving all JIT enable/disable decisions into
qv4global_p.h

Change-Id: I7eff5b4fbf1cd26297a08dee16984ad867358113
Reviewed-by: Lars Knoll <lars.knoll@digia.com>