x86/cpu: Renumber X86_VENDOR_* to form a bitmap

x86/cpu: Renumber X86_VENDOR_* to form a bitmap

CPUs from different vendors sometimes share characteristics.  All users of
X86_VENDOR_* are now direct equal/not-equal comparisons.  By expressing the
X86_VENDOR_* constants in a bitmap fashon, we can more concicely and
efficiently test whether a vendor is one of a group.

Update all parts of the code which can already benefit from this improvement.

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