KVM: x86: Advertise support for LKGS

KVM: x86: Advertise support for LKGS

Advertise support for LKGS (load into IA32_KERNEL_GS_BASE) to userspace
if the instruction is supported by the underlying CPU.

LKGS is introduced with FRED to completely eliminate the need to swapgs
explicilty.  It behaves like the MOV to GS instruction except that it
loads the base address into the IA32_KERNEL_GS_BASE MSR instead of the
GS segment’s descriptor cache, which is exactly what Linux kernel does
to load a user level GS base.  Thus there is no need to SWAPGS away
from the kernel GS base.

LKGS is an independent CPU feature that works correctly in a KVM guest
without requiring explicit enablement.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Link: https://lore.kernel.org/r/20250626173521.2301088-1-xin@zytor.com
Signed-off-by: Sean Christopherson <seanjc@google.com>