add FreeBSD/arm64 to packaging targets

[evgeni]

PR Description

Add FreeBSD/arm64 to packaging targets

Which issue(s) this PR fixes

Notes to the Reviewer

It builds fine on my 14.0-RELEASE system:

freebsd@generic:~/agent % ./dist/grafana-agent-freebsd-arm64 --version
agent, version freebsd-arm64-396c6ad-WIP (branch: freebsd-arm64, revision: 396c6ad08)
  build user:       freebsd@generic
  build date:       2024-03-22T10:13:27Z
  go version:       go1.22.0
  platform:         freebsd/arm64
  tags:             netgo,builtinassets

But I couldn't manage to cross-compile it from amd64. Are the drone builders you use native arm64? Or do they cross-compile?

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[CLAassistant]

All committers have signed the CLA.

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[github-actions]

This PR has not had any activity in the past 30 days, so the needs-attention label has been added to it.
If you do not have enough time to follow up on this PR or you think it's no longer relevant, consider closing it.
The needs-attention label signals to maintainers that something has fallen through the cracks. No action is needed by you; your PR will be kept open and you do not have to respond to this comment. The label will be removed the next time this job runs if there is new activity.
Thank you for your contributions!