Yeah I’m not sure why linuxbsd isn’t listed as an available platform, did you install a mold version for x86_64? did you install glibc for x86_64? A virtual machine sounds like a good path to have a stable linux environment.
The other platforms do not have a RISC-V section because so far linux is the only operating system that has been built for RISC-V. RISC-V is an architecture like ARM64 or x86_64, it is not a platform by itself. Cross compiling linux is not related to, nor does it require RISC-V.