x86, cpu: Add SMEP CPU feature in CR4

Add support for newly documented SMEP (Supervisor Mode Execution Protection)
CPU feature in CR4.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
LKML-Reference: <1305683069-25394-3-git-send-email-fenghua.yu@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>