x86: Add STIBP feature enumeration

x86: Add STIBP feature enumeration

Enumerate single thread indirect branch predictors (STIBP) feature. It
provides means to prevent indirect branch predictions from being
controlled by sibling HW thread.

Orabug: 27344012
CVE: CVE-2017-5715

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>