eal/x86: fix memcpy AVX-512 enablement

eal/x86: fix memcpy AVX-512 enablement

When testing on some x86 platforms, code compiled with meson was observed
running at a different power-license level to that compiled with make. This
is due to the fact that meson auto-detects the instruction sets available
on the system and enabled AVX512 rte_memcpy when AVX512 was available,
while on make, a build time AVX-512 flag needed to be explicitly set to
enable that AVX512 rte_memcpy code path.

In the absence of runtime path selection for rte_memcpy - which is
complicated by it being a static inline function in a header file - we can
fix this behaviour regression by similarly having a build-time option which
must be set to enable the AVX-512 memcpy path.

Fixes: a25a650be5f0 ("build: add infrastructure for meson and ninja builds")
Fixes: 3e1bb55fd6ef ("build/x86: add SSE flags")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Yingya Han <yingyax.han@intel.com>