Continuous integration for ARM64

We don't have ARM64 hosts in the Google Cloud, so we can't run tests on arm64 in kokoro like we run them on x86.

I see a few options what we can do here:
1. We can cross-compile gvsior for arm64 in kokoro. In this case, we will not be able to run tests.
2. We can run tests from kokoro on remote arm64 hosts. We need to find arm64 tests hosts somewhere. @lubinszARM, maybe you can help us with this?
3. Travis-CI added support for arm64 recently. We can build and run a minimal set of tests there:
https://travis-ci.org/avagin/gvisor/jobs/611633485?utm_medium=notification&utm_source=github_status
https://github.com/avagin/gvisor/commit/e3981b813a3c1fd0d7d9a666594b526d791113fc