dav1d-debian-unstable: Switch to cross-compiling libc instead of multi-arch
Since riscv64 is a relatively new architecture, there are still some growing pains. Multi-arch is currently not fully possible due to version mismatches wrt binNMU [1].
Since we only depend on libc for the main functionality,
libc6-dev-riscv64-cross should suffice.
As a downside, we now need to specify the library path for qemu-user
with -L /usr/riscv64-linux-gnu/ or the QEMU_LD_PREFIX env var.