Fix Apple Silicon on Xcode 12 GM

[designatednerd]

Related to #1280. In testing for the Xcode 12 GM everything's working swimmingly via my Intel mac but the DTK is throwing fits that I can't seem to solve at the moment, mostly related to unable to load standard library issues.

[designatednerd]

OK well it turns out I am both less dumb than I originally thought for not being able to get this to work, and dumber than I thought for not realizing I shouldn't download the GM on the DTK.

The Apple Silicon stuff was not included with the GM since that ships to the public, and AS stuff should still be using the beta track (still Xcode 12b6 for now, probably a 12.1 beta sometime soon).

Closing this out since everything still works on the most recent version of Xcode to support Apple Silicon, 12.0b6.