I started the second thread koraks mentioned.
Here are the key folders and files I made to get me started. This allowed me to compile into my IDE and make it work with various simple libraries for sensors. I did not fully package it to run with the Arduino IDE. I made the basic analog and digital read /writes plus SPI.
arduinocore\include
arduino.h
Print.h
Printable.h
wiring.h
wiring_digital.h
Wmath.h
WString.h
arduinocore\src\avr
pgmspace.h
arduinocore\src
Print.cpp
wiring.cpp
wiring_digital.cpp
Wmath.cpp
arduinovariant\include
variant.h
I did not fit the SPI into the proper folders.
SPI.cpp
SPI.h
I would have to give thanks to @pert who wrote a lot of text.