x86: Add GBPAGES CPUID macro, clean up CPUID comments

Add a GBPAGES CPUID macro for a future NPT test and reorganize the
entries to be explicitly Basic vs. Extended, with a hint that Basic leafs
come from Intel and Extended leafs come from AMD.  Organizing by Intel
vs. AMD is at best misleading, e.g. if both support a feature, and at
worst flat out wrong, e.g. AMD defined NX and LM (not sure about RDPRU,
but avoiding such questions is the whole point of organizing by type).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622210047.3691840-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>