On the way to fixing #3425, there are a few areas in which cfg generation is quite difficult to reason about. I'll be making a few changes here to simplify and make it more targeted so that we can get more accurate cfg generation for feature-gating.

Previously, all of the cfg attributes were calculated and written together. The trouble with this is that the arch cfg is applied to C++ types regardless while the remaining cfg attributes are applied to all types but only during package generation as it relates to feature-gating. This way each can be tailored and simplified and only calculated when actually needed.