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.

[1] https://wiki.debian.org/binNMU