Add aarch64 release

[troibe]

The opentitan project uses GoogleCloudPlatform/kms-integrations to retrieve its firmware signing keys from Google Cloud KMS.
We started adding support for ARM/aarch64 but one missing piece is GoogleCloudPlatform/kms-integrations.

I was easily able to build with bazel build //kmsp11/main:libkmsp11.so as described here and then upload a release to my own repo.
libkmsp11.so works as expected when used with the opentitan project on my ARM machine.

If GoogleCloudPlatform/kms-integrations was using GitHub actions for the releases I would just open a PR here.
Unfortunately I don't have enough insight how the Kokoro build/release system works.

Could you either guide me how I could open a PR to add the aarch64 release to this repo
or if that is not possible for external contributors add the release yourself?

[tdbhacks]

Apologies for the late reply! Yeah, we use Kokoro to run internal CI and build releases, though IIRC it doesn't support ready-to-go aarch64 images yet, so we'd have to maintain our own aarch image and set up a new pool + release flow.

Sadly, new supported platforms are generally a non-trivial amount of work, which would need to be prioritized internally first. This makes sense as a feature request though.

[matera-davcor]

We are currently migrating our signing workloads to ARM-based instances to optimize costs and performance, but the lack of an official aarch64 release for kmsp11 is a blocker for our HSM integration.

Since @troibe has already verified the build process with Bazel, could the team provide an update on the internal prioritization of this platform? If a full Kokoro pipeline is not yet feasible, would you consider a community-driven PR for a GitHub Actions-based release flow as an interim solution?