Well, I am officially giving up on trying to set up a bare metal development environment for the Teensy 4.0, at least for now. I believe it is possible, but I'd rather not fight against the platform too hard. If I gave it some more time, I could probably reasonably set up and use NXP's MCUXpresso, but just using GNU Arm and Makefiles seems extra challenging.
As much as I hate the Arduino IDE, if it works, it works, and it's the path of least resistance.
@twylo Man, if I had a nickel...
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko