Add an ARM build

[alexw91]

Find a way to get an ARM build working.

There are a couple of ways that we might be able to do it:

1. Cross compile an ARM binary on x86_64 Linux, without running any tests. This will just verify that we can compile and don't break the ARM build, which will get us most of the way.

2. Emulate an ARM build on Travis using qemu:

   • Support Android / Arm travis-ci/travis-ci#3376
   • https://www.tomaz.me/2013/12/02/running-travis-ci-tests-on-arm.html

3. AWS CodeBuild + EC2 ARM Processors:

   • https://aws.amazon.com/blogs/aws/new-ec2-instances-a1-powered-by-arm-based-aws-graviton-processors/

[dougch]

CodeBuild has support for ARM on AmazonLinux 2.0 as another option

[raycoll]

It would be great to have this in s2n's CI build now that AWS has Gravitron instances generally available and increasingly used

[dougch]

Covered in #2976