x86, cpufeature: Add cpufeature flag for SMEP

Add support for newly documented SMEP (Supervisor Mode Execution Protection) CPU
feature flag.

SMEP prevents the CPU in kernel-mode to jump to an executable page
that has the user flag set in the PTE.  This prevents the kernel from
executing user-space code accidentally or maliciously, so it for
example prevents kernel exploits from jumping to specially prepared
user-mode shell code.

[ hpa: added better description by Ingo Molnar ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
LKML-Reference: <1305683069-25394-2-git-send-email-fenghua.yu@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>