main/linux-lts: enable qemu guest support for riscv64

This enables the relevant modules to boot as a qemu guest (built-in/module matches our aarch64 config).

This has been tested with a self-built standard.iso, virtio disks, and uboot passed as kernel. My qemu command line:

qemu-system-riscv64 -M virt -m 512 -kernel alpine-uboot-241221-riscv64/u-boot/qemu-riscv64_smode/u-boot.bin -display none -serial stdio -device virtio-blk-pci,drive=cd0 -drive file=alpine-standard-241222-riscv64.iso,readonly=on,id=cd0