Motivation for this change

Adds a derivation for riscv-gnu-toolchain (https://github.com/riscv/riscv-gnu-toolchain) which is a GNU toolchain for RISC-V, including GCC. The derivation currently builds a Linux multilib (riscv64 and riscv32) Newlib cross-compilation toolchain oriented toward embedded systems applications, though one could add options to the derivation to build Linux/glibc and single-arch toolchains.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• [N/A] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• [N/A] Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.