buildbot-vm.sh: use QEMU bundled with cuttlefish for arm64 as well.

Previously we used it only for riscv64, but there's no reason not to use
it for arm64 - it's a more up-to-date version of QEMU which incorporates
recent fixes compared to the QEMU provided by the host system.

Test: on master-art manifest, ensure that the arm64 VM boots:
    lunch armv8-trunk_staging-eng

    export ART_TEST_SSH_USER=ubuntu
    export ART_TEST_SSH_HOST=localhost
    export ART_TEST_SSH_PORT=10001

    art/tools/buildbot-vm.sh create
    art/tools/buildbot-vm.sh boot

Change-Id: I0284cd09a1a618cf38a1b959744abcb6914e0871