Build as a universal binary on MacOSX.

[jemc]

trafficstars

WIP

[SeanTAllen]

How would this work with the runtime where we need both the X86 and Arm code which often defines the same datastructures in different forms?

[jemc]

It's totally possible to deal with that by basically just building everything twice and using lipo to stitch them together into the two slices of a "universal binary". However, it may not be ergonomic in our current build system to do that.

Anyway, please treat this draft PR as exploratory while I figure out not only what is possible, but also what is feasible in our build system.