Ah,
> 4. compile it and run
>
> cd xv6-riscv
> gmake TOOLPREFIX=/usr/local/nbtools/riscv64--netbsd/bin/ \
> CC=/usr/local/nbtools/bin/riscv64--netbsd-gcc qemu
that should be:
PATH=/usr/local/nbtools/bin:$PATH \
gmake TOOLPREFIX=riscv64--netbsd- qemu
-mochid