x86 CPU features detection for applications (and AMX)

[Florian Weimer]

* Dave Hansen:

> That's kinda my whole point.
>
> These *MUST* be curated to be meaningful.  Right now, someone just
> dumped a set of CPUID bits into the documentation.
>
> The interface really needs *three* modes:
>
> 1. Yes, the CPU/OS supports this feature
> 2. No, the CPU/OS doesn't support this feature
> 3. Hell if I know, never heard of this feature
> 	
> The interface really conflates 2 and 3.  To me, that makes it
> fundamentally flawed.

That's an interesing point.

3 looks potentially more useful than the feature/usable distinction to
me.

The recent RTM change suggests that there are more states, but we
probably can't do much about such soft-disable changes.

Thanks,
Florian