x86: Use clflushopt in clflush_cache_range

If clflushopt is available on the system, use it instead of clflush in
clflush_cache_range.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Link: http://lkml.kernel.org/r/1393441612-19729-3-git-send-email-ross.zwisler@linux.intel.com


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