emulator_arm64: Don't support AArch32

This is needed for the emulator to run on Apple Silicon.

In addition, we're going to move to 64-bit only soon across the platform
so it makes more sense to go with 64-bit-only going forward here.

Change-Id: I0d9d189cd0b7a07d6c315e8c0f99c7b4766b4bde