The most powerful, fully open, fully programmable desktop computer today is the Raptor Computing Systems Talos II workstation. It uses IBM OpenPOWER CPUs and the OpenBMC board firmware.
It makes no sense to use the x86 ABI today. 32-bit processors are not produced anymore (since post Pentium 4/~10 years ago). 64 bit mode is much better and even provides a better 32-bit ABI. The improvement in GCC 5 should be seen as an improvement for legacy software.
As an analogy, seeing it as a "refresh" for the old x86 ABI is like a patient that goes to the doctor and says he cuts his wrists every week and the doctor solution is to give him iron supplements.
https://www.raptorcs.com/content/TL2DS1/intro.html
Openess might be impacted by add-on hardware. For servers, more powerful OpenPOWER systems are available from IBM.