Log output on riscv5: https://tpaste.us/Lydm Near identical errors are produced on aarch64, loongarch64 and ppc64le.
(It usually helps if you post the actual build error.) It still works for me with the most modern Linux build environments, on archs including ppc64le, s390x, aarch64. But riscv64 support isn't yet in those files. Anyway, can you not just recreate those files yourself using autoreconf -fiv? That works fine for me, albeit with a ton of warnings, mostly about outdated syntax. Note that this project hasn't been maintained since 2007, so don't expect any updates.
Outdated config.guess and config.sub file in the source of project FuseISO.
Support for nrg with mixed data
fuseiso infinite loop parsing options for arm64