Add arm64 support

[julianfairfax]

This is an updated #393, which was closed because there is no desire to switch back to unpacking debs just to add arm64 support. While I understand the motivation between not going back to unpacking debs, this would close #167, and the current build method is from binaries anyway (which Element doesn't seem to be interested in building for arm64), so neither are built from source.

[flathubbot]

Started test build 105061

[flathubbot]

Build 105061 failed

[flathubbot]

Started test build 105063

[flathubbot]

Build 105063 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/87868/im.riot.Riot.flatpakref

[flathubbot]

Started test build 105065

[flathubbot]

Build 105065 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/87870/im.riot.Riot.flatpakref

[SISheogorath]

Was superseded by #451