Guests X86_BUG_NULL_SEG if and only if the host has them.  Use the info
from static_cpu_has_bug to form the 0x80000021 CPUID leaf that was
defined for Zen3.  Userspace can then set the bit even on older CPUs
that do not have the bug, such as Zen2.

Do the same for X86_FEATURE_LFENCE_RDTSC as well, since various processors
have had very different ways of detecting it and not all of them are
available to userspace.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>