ppc: Move load and store helpers, switch to AREG0 free mode

ppc: Move load and store helpers, switch to AREG0 free mode

Add an explicit CPUPPCState parameter instead of relying on AREG0
and rename op_helper.c (which only contains load and store helpers)
to mem_helper.c. Remove AREG0 swapping in
tlb_fill().

Switch to AREG0 free mode. Use cpu_ld{l,uw}_code in translation
and interrupt handling, cpu_{ld,st}{l,uw}_data in loads and stores.

Signed-off-by: Blue Swirl <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>