In trying to fix the makefile to run with CrossPack on OS X; I noticed that system.c is not being compiled by the makefile; and attempting to add it to the makefile causes compilation to fail (the hardware version macro fails to expand). However, omitting it does not cause any link errors, so...
tinyG is very popular, but few have need (or capability/desire) to customize/port as you are attempting.
And perhaps I should observe that folks able to dig about in the weeds at that level don't come here for help.
Those seeking the bleeding edge tinyG experience on new hardware tend to head down the tinyG2 path.
The issues link above is appropriate should you find inconsistencies in the source tree or makefiles, etc. This forum is more for operational issues when using 'standard' tinyG hardware and published binaries.