size: move the size regression job to arm64

Currently the size regression job does runs on x86 Ubuntu with gcc and ld. Once https://github.com/Microsoft/azure-pipelines-agent/issues/1911#issuecomment-433957452 is resolved move the job to arm64 machines with clang and lld to closely resemble the size analysis [doc requirements](https://envoy-mobile.github.io/docs/envoy-mobile/latest/development/performance/binary_size.html#getting-a-binary).

Start by locating the current size regression job configuration and reviewing the linked Azure Pipelines issue that blocks the move. Compare the job with the binary size documentation requirements. Done means the job runs on arm64 machines with clang and lld and reflects the documented setup.

Written by the indexing model from the issue text.