ofborg should be able to treat aarch64-darwin as x86_64-darwin for builds

[cole-h]

The M1s have Rosetta 2, which we can leverage to get extra builds going for x86_64-darwin.

This is inspired by the fact that we had only one x86_64-darwin builder for a while -- one of the x86_64 machines is currently unavailable, one was stuck on a build for over a month, somehow, and one has stopped responding to pings / SSH altogether.

One idea I've had is to just special-case aarch64-darwin in the builder binary to also subscribe to the x86_64-darwin build queue. This is the ideal path, because it (likely) means that ofborg won't be trying to do 2 builds at the same time and potentially exhausting resources. If that falls through, another idea could be to just spawn another builder with a config claiming that it's x86_64-darwin.