x86/cpuid: detect null segment behaviour on Zen2 CPUs

x86/cpuid: detect null segment behaviour on Zen2 CPUs

All Zen2 CPUs actually have this behaviour, but the CPUID bit couldn't
be introduced into Zen2 due to a lack of leaves. So, it was added in a
new leaf in Zen3. Nonetheless, hypervisors can synthesize the CPUID
bit in software.

So, Xen probes for NSCB (NullSelectorClearsBit) and
synthesizes the bit, if the behaviour is present.

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