Add (native) ARMv7A and AArch64 CI runners

We need native CI runners to test arm/arm64 asm. CI runners with clang-5.0 are also required to enforce arm asm syntax restriction specified in Contributing.md. https://code.videolan.org/janne/dav1d/tree/test-ci-aarch64 adds arm64 CI runners using a private gitlab CI runner in my repository. We can move the runner over to this project or I can assist in recreating it on videolan infrastructure.

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