Add build for Linux on Arm64

[mikhail-vl]

What this PR does / why we need it:

You can run Grafana on arm64 and should be able to run backend plugins.
Maybe we should include darwin on arm64 to be ready for the upcoming Mac?

Which issue(s) this PR fixes:

Fixes #220

Special notes for your reviewer:

Tested on Khadas VIM3 Pro!

[aknuds1]

@ryantxu @marefr @kylebrandt What do you think about including ARM64 as a Mage target?

Should it be built by default (as the PR proposes)?

[gytisrepecka]

@ryantxu @marefr @kylebrandt What do you think about including ARM64 as a Mage target?

Should it be built by default (as the PR proposes)?

Are all default build targets included into plugin package that is installed with grafana-cli plugins install?

Maybe it's not the right place to ask, but I was wondering why, say, on ARM64 machine I also get all other binaries?

[aknuds1]

@gytisrepecka It depends on how the individual plugin is packaged, but I don't see any reason at least why other architectures should get installed. For grafana-image-renderer f.ex. I only get Linux/AMD64 installed.

Here we're just talking about building, not packaging.

It was fixed.

[ryantxu]

LTGM -- yes, the packaging discussion is different, the first step is having it build

[ryantxu]

thank you @mikhailredis!