net/virtio: fix 32-bit build with clang 6

net/virtio: fix 32-bit build with clang 6

Clang 6.0.0 will undefine function _mm512_maskz_set1_epi64 on i686
target. Fix it by replacing the function with _mm512_set4_epi64 when
doing 32-bit build.

Warning message during build:
../drivers/net/virtio/virtio_rxtx_packed_avx.c:385:19: warning:
implicit declaration of function '_mm512_maskz_set1_epi64' is invalid
in C99 [-Wimplicit-function-declaration]

Fixes: 77d66da83834 ("net/virtio: add vectorized packed ring Rx")

Signed-off-by: Marvin Liu <yong.liu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>