x86/cpufeatures: Define X86_FEATURE_AMD_IBRS_SAME_MODE

Per the APM [1]:

  Some processors, identified by CPUID Fn8000_0008_EBX[IbrsSameMode]
  (bit 19) = 1, provide additional speculation limits. For these
  processors, when IBRS is set, indirect branch predictions are not
  influenced by any prior indirect branches, regardless of mode (CPL
  and guest/host) and regardless of whether the prior indirect branches
  occurred before or after the setting of IBRS. This is referred to as
  Same Mode IBRS.

Define this feature bit, which will be used by KVM to determine if an
IBPB is required on nested VM-exits in SVM.

[1] AMD64 Architecture Programmer's Manual Pub. 40332, Rev 4.08 - April
    2024, Volume 2, 3.2.9 Speculation Control MSRs

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20250221163352.3818347-2-yosry.ahmed@linux.dev
Signed-off-by: Sean Christopherson <seanjc@google.com>

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