RFA: RL78: Allow SP to be used as a base register

Hi DJ, Please may I apply the patch below. It fixes the RL78 backend so that the stack register can be used as a base address register. Tested with no regressions on an rl78-elf toolchain. Cheers Nick PS. I am currently investigating allow r8-r15 to be used as base registers.

Re: RFA: RL78: Allow SP to be used as a base register

Please may I apply the patch below. It fixes the RL78 backend so that the stack register can be used as a base address register. Yes, please. Thanks!