Grafana and mattermost are not compiled (pre-built binaries are used) but aarch64/arm64 case is not handle

Summary

While packaging gitlab for aarch64 (See https://gitlab.com/gitlab-org/omnibus-gitlab/issues/4942), I noticed that Grafana and Mattermost are not compiled but pre-built binaries are used. This is a problem since aarch64/arm64 case is not handle and thus x86_64 binaries are packaged for aarch64.

Proposal

We should use aarch64 binaries or recompile from sources for aarch64/arm64 packages.

References

Grafana provides aarch64 binaries at https://grafana.com/grafana/download?platform=arm so we can just use https://dl.grafana.com/oss/release/grafana-6.3.5.linux-arm64.tar.gz for aarch64

Mattermost does not provide pre-built binaries: https://mattermost.com/download/