Well, I'm sorry to have kept you guys waiting for so long, and the news is not good: so far as I can tell, and after exhaustive research, it seems that as of F12, I will not be able to build the Mach64 kmods.
"Fixing" the DRM and Mesa stuff to get the Mach64 3D driver is fine, but the kernel source code does not contain the code needed to build the kmods (kernel driver modules):
It simply is not there, as can be readily seen. Without the kmods, the 3D driver is useless.
Previously, these kmods were built from X.Org DRM (/mesa/libdrm/) source code (compiled against the kernel source), but this code branch ("linux-core") has been completely purged, and the code moved to the kernel source - or rather in this case: simply dropped. Much like a live hand grenade. I cannot help but suspect that this was [semi-]deliberate, or perhaps the result of sheer apathy.
These changes effectively end this little project of mine. Unless and until such time as the Mach64 kmod code is added back into the kernel source tree (which I seriously doubt will ever happen), I cannot see any way to proceed. I have neither the skill nor the motivation at this time to pursue the matter further.
Again, I apologize for taking so long to respond. I wish the news was better, but it is what it is.
Thanks to all who have expressed interest and support for this project. It was fun while it lasted.
P.S. I will continue to build 32-bit (non-PAE) kmods for F11 until end-of-life, and all files will remain available indefinitely, but support from me will end when F11 is officially end-of-life.