KVM: x86: Add a macro to init CPUID features that are 64-bit only

KVM: x86: Add a macro to init CPUID features that are 64-bit only

Add a macro to mask-in feature flags that are supported only on 64-bit
kernels/KVM.  In addition to reducing overall #ifdeffery, using a macro
will allow hardening the kvm_cpu_cap initialization sequences to assert
that the features being advertised are indeed included in the word being
initialized.  And arguably using *F() macros through is more readable.

No functional change intended.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-25-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>