Add support for testing with Graviton2

[janaknat]

Travis CI supports running on AWS Graviton2 instances. For more information please see: https://blog.travis-ci.com/2020-09-11-arm-on-aws

Travis CI build logs for arm64 and arm64-graviton2: https://travis-ci.com/github/janaknat/php-src/builds/226356460

[nikic]

What is the difference between the arm64 and the arm64-graviton2 archs? Is it a matter of arm64-graviton2 being faster? If that's the case, I think we can just replace the existing arm64 job with the arm64-graviton2 one, there would be no need to run both.

The main problem with our arm64 testing right now is that since Travis switched to the credit system, we only do a single nightly test run, while ideally we should be testing each PR/merge.

[janaknat]

Yes, arm64-graviton2 is faster. It is backed by Graviton2 instances.

As per https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing 'Building on a public repositories only' point 4, OSS projects can request for credits. It might help with the testing.

I can update the PR to remove the arm64 job and have the arm64-graviton2 job alone.

[nikic]

Okay, let's go with this variant then!