Support building/running prysm for aarch64-linux

[PoisonPhang]

Currently, prysm only targets x86_64-linux adding targets for aarch64-linux and more would be helpful.

[aldoborrero]

Hi, @PoisonPhang we haven't enabled support for other architectures yet as we haven't tested those so well (for myself, I'm only running on x86_64-linux for now), but we can add those pretty easily. I know Prysm supports aarch64-linux, so feel free to add a PR with the change.

Maybe we also need to tweak the build system with the recommended env variable export PRYSM_ALLOW_UNVERIFIED_BINARIES=1 when targeting arm as recommended on their FAQ.

[PoisonPhang]

Switching to Bazel is also in the near future, yes? @aldoborrero

If I can find the time - I might try and tackle both this issue and the bazel upgrade.

[aldoborrero]

In this comment, I leave the summary if it makes sense or not to switch to Bazel (in summary, probably not).

If you want, we can keep discussing bazel-related matters in #171 and I'll assign both issues to you.

[aldoborrero]

Closed by #386