x86: Fix cpu offline bug: add clflush inside dead loop

x86: Fix cpu offline bug: add clflush inside dead loop

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>