Changes from v1:
 * Fix non-x86 PECI compile issues by lifting some x86 macros
   into an arch-independent header.
 * Make intel_get_platform_id() match its name and return an ID,
   not a mask.
 * Sort #includes
 * Move ->x86_platform_id comment

The platform ID vs. platform mask confusion meant that v1
worked by total accident. If you tested v1, I'd really
appreciate you retest this as well.

--

There was a report[1] that CPUs running updated microcode were being
reported as running old microcode. The reason is that the old
microcode list neglects to take the platform ID into account.

The platform ID is an Intel-only construct that allows CPUs that
otherwise have the same model/family/stepping to take different
microcode revisions. The microcode loader itself already checks this.
Only the recent "old_microcode" checker failed here.

Treat the platform ID as a peer of model/family/stepping. Store it
in 'struct cpuinfo_x86', enable matching on it with with 'struct
x86_cpu_id', and flesh out the 'old_microcode' list with it.

This fixes the report of an inaccurate, false positive in the
'old_microcode' vulnerability file.

1. https://lore.kernel.org/all/38660F8F-499E-48CD-B58B-4822228A5941@nutanix.com/

Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: x86@kernel.org
Cc: Jon Kohler <jon@nutanix.com>