Adding RISCV from #1419

[fede2cr]

Copy paste from @xfbs, to fix bug #1419.

Built on Fedora-riscv64, and run the tests. It is working fine as a dependency for sccache, and compiled without any errors at all.

[briansmith]

I don't think this PR is enough to actually get RISC-V working because it is missing a solution to the bn_mul_mont problem. I outlined a more general solution in issue #1455 that should cover RISC-V, and I'm going to close this issue so we can focus on that more general approach.