Add support for riscv64 and riscv32

[luhenry]

The OpenJDK sets os.arch to riscv64 or riscv32. This trips the architecture detection as it assumes the os.arch=riscv on RISC-V. Adds support for riscv32 and riscv64 to return riscv regardless. The bitness will be detected in guessBitnessFromArchitecture

[trustin]

Thanks for the PR, @luhenry. I realized #62 is already filed by @zinovya. Let me merge his PR after adding support for riscv32 to it. One differnce is that riscv64 will be detected as riscv64 instead of riscv, because otherwise it will break the consistency, e.g. x86_32 vs x86_64.

[luhenry]

Perfect, thanks for merging it in, and thanks for taking care of this project! 🎉