buildtools: fix AVX512 check for Python 3.5
buildtools: fix AVX512 check for Python 3.5

Python 3.5 subprocess.run() does not have a capture_output
parameter (it is present only in 3.7 and up).
Capture output by using subprocess.PIPE for stdout instead.

Fixes: bb9cd91095b3 ("buildtools: make AVX512 check portable")
Cc: stable@dpdk.org

Signed-off-by: Lance Richardson <4e26776f5588f0e1c0c16cd2ddc0e9efc79c5b3a@broadcom.com>
Acked-by: Dmitry Kozlyuk <622f8c4298455ea0ac8bf96da29b254c86170938@gmail.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk