Draft: debian: Install RISCV64 cross-compiler

WIP

RISCV64 cross building/testing with the registry.gitlab.haskell.org/supersven/ci-images/x86_64-linux-deb12image:

configure && build

./boot
./configure --target=riscv64-linux-gnu CC=riscv64-linux-gnu-gcc
hadrian/build -j --docs=none --flavour=validate --bignum=native

Run tests

CROSS_EMULATOR="qemu-riscv64 -L /usr/riscv64-linux-gnu/" EXTRA_HC_OPTS='-fexternal-interpreter' hadrian/build -j2 --docs=none --flavour=validate --bignum=native --test-speed=fast test