RISC-V support for Gentoo Prefix

RISC-V is an emerging open standard instruction set architecture that can be used to build hardware without paying royalties. Gentoo Prefix develops and maintains a way of installing Gentoo systems in a non-standard location, designated by a "prefix". Prefix installs a complete Gentoo userspace into a directory by running the bootstrap-prefix.sh script. The script launches the interactive installer which will ask for some information and discuss some options based on the system being bootstrapped. Prefix has support for several architectures, and we plan to add support for RISC-V. We will make it possible to bootstrap, and use a Gentoo Prefix system on RISC-V architecture.