i386: Define AMD's no SSB mitigation needed.

i386: Define AMD's no SSB mitigation needed.

AMD future CPUs expose a mechanism to tell the guest that the
Speculative Store Bypass Disable is not needed and that the
CPU is all good.

This is exposed via the CPUID 8000_0008.EBX[26] bit.

See 124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf

A copy of this document is available at
        https://bugzilla.kernel.org/show_bug.cgi?id=199889

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Message-Id: <20180601153809.15259-3-konrad.wilk@oracle.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>