[Merge] livecd-rootfs:fix-riscv64-no-subarch into livecd-rootfs:ubuntu/master

[Philip Roche]

+1 based on the build errors we @ CPC were seeing 
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https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/427627
Your team Ubuntu Core Development Team is requested to review the proposed merge of livecd-rootfs:fix-riscv64-no-subarch into livecd-rootfs:ubuntu/master.