Comment # 3 on bug 1139856 from Richard Biener

cross-riscv64-elf-gcc9 contains static libgcc and uses newlib which you need to
install as well, cross-riscv64-newlib-devel (doesn't seem we put in Requires).

cross-riscv64-gcc9 is the usual crippled glibc based cross which can only
be used to build the kernel because it lacks a glibc for building and we
lack it for building glibc.

The crosses are essentially unmaintained, the other Andreas added them.

Setting up crosses for glibc based compilers properly will be hard, setting
them up for targets where OBS has builders might work in the hackish way
(package noarch cross-$arch-glibc-devel on native $arch by shuffling around
files and then aggregating them across the OBS repos).