Add riscv64 configurations to local vmtest.

We can now perform cross platform testing for riscv64 bpf using the
following command:

PLATFORM=riscv64 CROSS_COMPILE=riscv64-linux-gnu- vmtest.sh \
    -l ./libbpf-vmtest-rootfs-2024.08.30-noble-riscv64.tar.zst -- \
    ./test_progs -d \
        \"$(cat tools/testing/selftests/bpf/DENYLIST.riscv64 \
            | cut -d'#' -f1 \
            | sed -e 's/^[[:space:]]*//' \
                  -e 's/[[:space:]]*$//' \
            | tr -s '\n' ','\
        )\"
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-10-pulehui@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>