regcache: Code factorization / cleanup
Replace lightrec_free_regs by fine-grained frees using lightrec_free_reg
Modify to use latest Lightning API, remove some arch-dependent hacks
lightrec: Optimize address lookup block by using different registers

We now calculate the native address in JIT_R0, which is the return
register on MIPS and most probably other archs as well.
regcache: Add function lightrec_clean_reg
emitter: Store back JIT_R0 register before executing the delay slot