Add RISC-V instructions encoding.

This is the output of
$ cat opcodes opcodes-rvc-pseudo opcodes-rvc opcodes-custom |

./parse-opcodes -c

It is confirmed by author that the output of parse-opcodes is
in the public domain.

This will be required for DDB disassembler.

Discussed with: Andrew Waterman <waterman@eecs.berkeley.edu>
Obtained from: https://github.com/riscv/riscv-opcodes
Sponsored by: DARPA, AFRL