Cache the host CPU vendor for userspace and share it with guest code.

All the current callers of this_cpu* actually care about host cpu so
they are updated to check host_cpu_is*.
Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Vishal Annapurve <vannapurve@google.com>
Link: https://lore.kernel.org/r/20230111004445.416840-3-vannapurve@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>