No RISC-V configuration in the unstable tree

[Impact]

 * We would like to eventually build and test the unstable and generic
   kernels on the riscv64 architecture. Fixing this is a step towards
   that.

 * Having the riscv64 configuration alongside the other architectures
   should ensure it doesn't drift away from the generic configuration
   over time.

 * This allows us to do backports more easily.

[Test Plan]

 * After merging riscv64 the generated configuration for other
   architectures should not change.

 * The generated riscv64 configuration should be as close as possible to
   the current linux-riscv configuration. Right now unstable is based on
   5.19 while the riscv kernel is based on 5.18, so it can't be identical.

[Where problems could occur]

 * Adding the riscv64 annotations may subtly change annotations from other
   architectures.

 * Adding the riscv64 configuration and annotations might confuse tooling
   scripts that are not prepared to handle the new architecture.

[Other Info]

 * In the near future we'd also like to base the StarFive and Allwinner
   (Nezha) kernels on the same series that will become the generic kinetic
   kernel.