x86/cpuid: Introduce named feature bitfields

x86/cpuid: Introduce named feature bitfields

It greatly aids the readibility of code to express feature checks with their
direct name (e.g. p->basic.mtrr or p->extd.lm), rarther that by a field and a
bitmask.  gen-cpuid.py is augmented to calculate a suitable declaration to
live in a union with the underlying feature word.

gen-cpuid.py doesn't know Xen's choice of naming for the feature word indicies
(and arguably shouldn't care), so provides the declarations in terms of their
numeric feature word index.  The DECL_BITFIELD() macro (local to cpuid_policy)
takes a feature word index name and chooses the right declaration, to aid
clarity.

All X86_FEATURE_*'s are included in the naming, other than the features
fast-forwarded from other state (APIC, OSXSAVE, OSPKE), whose value cannot be
read out of the feature word.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>