Add Arm64 as target build platform

[dneary]

It would be really nice if bpftop were to build native Arm64 binaries in addition to x86. All of the eBPF tooling being measured and instrumented is available on Arm64, and as a Rust program, there should be no porting issues.

[jfernandez]

Hi @dneary, I'll look into this for the next release.

[jfernandez]

I added the arm64 target for building with cross-rs. I updated the README with instructions on how to build the binary. For now we'll just publish the x86 bins in the release, but you can now build it locally. Sometime in the future I'll get to updating the build logic to publish both bins.

[dneary]

Thanks Jose! If it's any help, I may be able to help ensure compute resources for build & test.