At some platform (like Xen 7400), when hyperthreading, an offlined
thread may waked spuriously up by its brother, and returning around
the loop.  This patch explicitly clflush the cache line in a light
weight way to workaround potential issue.  Unlike wbinvd, clflush is
not serializing instruction, hence memory fence is necessary to make
sure all load/store operation visible before flush cache line.

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>