Enable RISC-V support in Debian

More information: https://wiki.debian.org/RISC-V.

First, upgrade to testing with ports support, then do (as root):

su -

apt install -y qemu-system-misc qemu-user-static binfmt-support
dpkg --add-architecture riscv64
apt-get install -y gcc-riscv64-linux-gnu g++-riscv64-linux-gnu
cat >/etc/ld.so.conf.d/riscv64-linux-gnu.conf <<EOF
/usr/local/lib/riscv64-linux-gnu
/lib/riscv64-linux-gnu
/usr/lib/riscv64-linux-gnu
/usr/riscv64-linux-gnu/lib/
EOF
ln -s /usr/riscv64-linux-gnu/lib/ld-linux-riscv64-lp64d.so.1 /lib