eal: add cache line demotion API

eal: add cache line demotion API

rte_cldemote is similar to a prefetch hint - in reverse.
On x86, cldemote(addr) enables software to hint to hardware that line is
likely to be shared. This is quite useful in core-to-core communications
where cache-line is likely to be shared.
ARM and PPC implementation is provided with NOP and can be added if any
equivalent instructions could be used for implementation on those
architectures.

Signed-off-by: Omkar Maslekar <omkar.maslekar@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>