`font-subset` should run natively on Apple Silicon arm machines

The `bin/cache/artifacts/engine/darwin-x64/font-subset` artifact only contains a x64 slice, so runs with Rosetta on an arm64 Mac.  Ship a native arm64 variant to run on Apple Silicon host machines.

https://github.com/flutter/engine/blob/82c2a5dcfa8c9cac6cc39898b808797f0bc43f47/tools/font-subset/BUILD.gn