build: enable ARM NEON flag when __aarch64__ defined

build: enable ARM NEON flag when __aarch64__ defined

GCC version 4.8.5 does not pre-define __ARM_NEON. NEON is not
optional for ArmV8. Hence NEON related code can be enabled
when __aarch64__ is defined.

Bugzilla ID: 82
Cc: stable@dpdk.org
Reported-by: Raslan Darawsheh <rasland@mellanox.com>
Reported-by: Thomas F Herbert <therbert@redhat.com>
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>