[CI] Cross build for Linux ARM and AArch64

Summary:
As per title. There is no easy way to run the tests, so this is build
only.

Test Plan: Run the build-linux-arm and build-linux-aarch64 builds.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5853