Add support for the RISC-V architecture and configs/nr5m100-nexys4 board. I...

Add support for the RISC-V architecture and configs/nr5m100-nexys4 board.  I will be making the FPGA code for this available soon (within a week I would say).  The board support on this is pretty thin, but it seems like maybe a good idea to get the base RISC-V stuff in since there are people interested in it.