Cross builds aren't really a supported way to build RPMs. No other architecture supported by Fedora uses cross-compilation, so it would just be an endless stream of problems and forks.

The current autobuilder is using QEMU (without virtio, but it'd be a lot easier and faster with) running on a 16 core Xeon. https://github.com/rwmjones/fedora-riscv-autobuild

But for the Fedora official builders, it's 19" rackable RISC-V real server hardware, and that's the non-negotiable condition we've been given, which I support and understand.

It'll be a few years, but it doesn't stop people using Fedora on RISC-V right now, they just have to grab the packages from a slightly different URL: https://fedorapeople.org/groups/risc-v/