i386: Add spec-ctrl CPUID bit

Add the feature name and a CPUID_7_0_EDX_SPEC_CTRL macro.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20180109154519.25634-4-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>