i386: define the AMD 'amd-ssbd' CPUID feature bit

i386: define the AMD 'amd-ssbd' CPUID feature bit

AMD future CPUs expose _two_ ways to utilize the Intel equivalant
of the Speculative Store Bypass Disable. The first is via
the virtualized VIRT_SPEC CTRL MSR (0xC001_011f) and the second
is via the SPEC_CTRL MSR (0x48). The document titled:
124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf

gives priority of SPEC CTRL MSR over the VIRT SPEC CTRL MSR.

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

Anyhow, this means that on future AMD CPUs there will be  _two_ ways to
deal with SSBD.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Message-Id: <20180601153809.15259-2-konrad.wilk@oracle.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>