Add arm64 builds to Drone

[webvictim]

Uses an a1 machine in AWS I set up a while ago to build arm64 Teleport binaries for Linux.

$ file ./teleport/teleport
./teleport/teleport: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, BuildID[sha
1]=ee9a95dfd2f09d6e25b064bfda88d97c39423089, for GNU/Linux 3.7.0, stripped

$ ./teleport/teleport version
Teleport v5.0.0-dev.15 git:v5.0.0-dev.15-0-g96583a89c go1.14.4

Needs some accompanying changes in Houston to display downloads to the public as well.

Fixes #1812

[benarent]

Will this just build the latest, or will we also build for 4.0,4.1,4.2,4.3

[webvictim]

This PR is only going into master currently so it'd start with the next release branch we cut (5.0.0)

We could backport it to 4.4 easily so we'd get arm64 from 4.4.2 onwards if we wanted.

4.3 is trickier but we could cut and make a specific 4.3.8 release to add arm64 if we wanted.

Anything before that would require changes to Jenkins so not happening.

[benarent]

Thanks for the clarification Gus, for now let's just back port to 4.3 ( as we've a customer request for this ) .

[benarent]

Out of interest, What size A1 instance are we using and how long does it take?

[webvictim]

https://drone.gravitational.io/gravitational/teleport/2663/1/3

It's currently an a1.large so 2 vCPUs, 4GB RAM. The build stage took 4m17s. We could probably go down to an a1.medium to halve the cost if we wanted - AMI builds take way longer than artifacts currently.

[MiLk]

Thanks for the work here !

If cost is an issue, wouldn't a t4g instance be better?

[webvictim]

@awly @r0mant @russjones Ping for CODEOWNERS approval please.

[webvictim]

We're not super concerned about the cost for now - fast builds are more important. Will keep notes for the future though :)