[dpdk-stable] AVX512 bug on SkyLake

[Thomas Monjalon]

Below is my conclusion for this bug.
An expert of x86 is required to follow-up.

Summary:
	- CPU: Intel Skylake
	- Linux environment: Ubuntu 18.04
	- Compiler: GCC 7 or 8
	- Scenario: testpmd crashes when it starts forwarding
	- Behaviour: AVX2 version of rte_memcpy() fails if optimized for AVX512
	- Context: inline rte_memcpy() is called from
			inline rte_mempool_put_bulk(), called from
			mlx5_tx_complete() (inline or not)
	- Analysis: AVX512 optimization changes vmovdqu to vmovdqu8

Latest status can be found in Bugzilla:
	https://bugs.dpdk.org/show_bug.cgi?id=97#c35