x86/cpu, kvm: Move X86_FEATURE_LFENCE_RDTSC to its native leaf

The LFENCE always serializing feature bit was defined as scattered
LFENCE_RDTSC and its native leaf bit position open-coded for KVM.  Add
it to its newly added CPUID leaf 0x80000021 EAX proper.  With
LFENCE_RDTSC in its proper place, the kernel's set_cpu_cap() will
effectively synthesize the feature for KVM going forward.

Also, DE_CFG[1] doesn't need to be set on such CPUs anymore.

  [ bp: Massage and merge diff from Sean. ]

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230124163319.2277355-5-kim.phillips@amd.com