debian12: Install addtitional toolchain for LoongArch64

Use tools that are not officially released to facilitate CI testing.

Reference: 6efac743

Addtional tools:

cross-tools: https://github.com/loongson/build-tools/releases/download/2025.02.21/x86_64-cross-tools-loongarch64-binutils_2.44-gcc_14.2.0-glibc_2.41.tar.xz

qemu: https://github.com/loongson/build-tools/releases/download/2025.02.21/qemu-loongarch64

Usage:

Based on registry.gitlab.haskell.org/supersven/ci-images/x86_64-linux-deb12, add additional cross-tools and qemu.

Building:

./boot
./configure --target=loongarch64-unknown-linux-gnu --with-intree-gmp
hadrian/build -j16 --docs=none

Testing:

CROSS_EMULATOR="qemu-loongarch64 -L /opt/cross-tools/target/" EXTRA_HC_OPTS='-fexternal-interpreter' hadrian/build -j16 --docs=none --flavour=validate --bignum=native --test-speed=fast test`