Add aarch64 release

The [opentitan project](https://github.com/lowRISC/opentitan) uses `GoogleCloudPlatform/kms-integrations` to retrieve its firmware signing keys from Google Cloud KMS.
We started adding [support for ARM/aarch64](https://github.com/lowRISC/opentitan/pull/26476) but one missing piece is `GoogleCloudPlatform/kms-integrations`.

I was easily able to build with `bazel build //kmsp11/main:libkmsp11.so` as described [here](https://github.com/GoogleCloudPlatform/kms-integrations/blob/master/BUILDING.md) and then upload a [release to my own repo](https://github.com/troibe/kms-integrations/releases/tag/pkcs11-v1.6).
`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?