net/i40e: add option to use latest vector path

net/i40e: add option to use latest vector path

For IA, the AVX2 vector path is only recommended to be used on later
platforms (identified by AVX512 support, like SKL etc.) This is because
performance benchmark shows downgrade when running AVX2 vector path on
early platform (BDW/HSW) in some cases. But we still observe perf gain
with some real work loading.

So this patch introduced the new devarg use-latest-supported-vec to
force the driver always selecting the latest supported vec path. Then
apps are able to take AVX2 path on early platforms. And this logic can
be re-used if we will have AVX512 vec path in future.

This patch only affects IA platforms. The selected vec path would be
like the following:
  Without devarg/devarg = 0:
  Machine	vPMD
  AVX512F	AVX2
  AVX2	SSE4.2
  SSE4.2	SSE4.2
  <SSE4.2	Not Supported

  With devarg = 1
  Machine	vPMD
  AVX512F	AVX2
  AVX2	AVX2
  SSE4.2	SSE4.2
  <SSE4.2	Not Supported

Other platforms can also apply the same logic if necessary in future.

Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>