I can almost bet,
"ARM K12 or K13 is live or it's already happened".Do you remember AMD Threadripper story?
if you only intent to build an ARM CPU, it is not really a challenge. Just take an ARM reference design and select a PDK for it and go from there. For someone like AMD it is piece of cake.
Most of the work done by all our SoC vendors, who supply us ARM SoCs is actually building the stuff around it.
DSPs, peripherals, clock trees, memory subsystems, and the big elephant, baseband IP, etc are which are not part of ARM offerings.
In fact Baseband is the main thing that is eleminating many players like NV for example from the game.
Talking to ARM SoC vendors is one of the responsibilities in my day job, we have a few suppliers and we work with them constantly to industrialize our products.
SoCs are a different game from CPUs. You have to continuously support your customer with BSP, Firmwares, drivers, Audio Frameworks, DSP toolkits, board designs and the like. Nothing is standardized.
if AMD decides to, getting X1 to market is easy peasy, question is what more do they want from it. How far do they want to tweak the architecture.
Snapdragon and Exynos for example carries mainly vanilla ARM cores with minor tweaks. They stopped developing their own cores a while ago.
If available I would be interested in an ARM desktop CPU which I can run Linux and Windows. I had a chance to work with Centriq some time ago, but lots of hair pulling to get stuff working.
On Linux, you really have to pull hairs to get lots of stuff working with the ARM SBCs that I have personally for my private projects. Not sure about Windows.
Most of the stuff to get it working (besides launching a console) is closed source, good luck getting stuff to run without any developer/customer account.