Cherry-pick #23479 to 7.x: Support for linux/arm64

[michalpristas]

Cherry-pick of PR #23479 to 7.x branch. Original message:

What does this PR do?

Just adds a correct arch for arm64 so arm64 beats are downloaded.
Tested on AWS arm64 server, dashboards seemed ok.

Why is it important?

support Arm 64 packages

How to test

Ideally you have an arm64 device or libvirt enabled and it's easy.
What i did is that I

• built binary using DEV=true SNAPSHOT=TRUE PLATFORMS="linux/arm64" mage package command
• used my AWS subscription to spin-up ARM64 Ubuntu server,
• scp-ed arm packaged there
• ssh, untar and clear data/elastic-agent-*/downloads directory
• install with fleet 8.0.0-SNAPSHOT
• everything should be ok and dashboards populated

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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Build stats

• Build Cause: Pull request #23522 opened

• • Start Time: 2021-01-15T09:03:17.803+0000

• Duration: 17 min 19 sec

• Commit: af5ad86

Test stats 🧪

Test	Results
Failed	0
Passed	1414
Skipped	6
Total	1420

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	1414
Skipped	6
Total	1420

[blakerouse]

Backport looks good.