x86/xsaves: Enable xsaves/xrstors

If xsaves/xrstors is enabled, compacted format of xsave area will be used
and less memory may be used for context per process. And modified
optimization implemented in xsaves/xrstors improves performance of saving
xstate.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1401387164-43416-16-git-send-email-fenghua.yu@intel.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>