Switch Cirrus CI to aarch64 for macOS targets

Cirrus CI is killing support for x86_64 macOS targets in 6 weeks time:

https://cirrus-ci.org/blog/2022/11/08/sunsetting-intel-macos-instances/

all docs about them have been removed, and everything exclusively refers to the aarch64 targets instead.

lcitool has blindly assumed Cirrus CI was always x86_64 based. Fortunately not too much needs changing, we just add an 'arch' field in the metadata and tweak some hardcoded arches.

The choice of architecture is determined implicitly by the image path used for the cirrus job.

Technically Cirrus CI supports both x86_64 and aarch64 for the Monteray release, but given all x86_64 support is being killed in 6 weeks, I decided NOT to support multiple native architectures per Cirrus target, to keep life a little simpler.