build: set -mfpu=neon flag for armv7-a with meson

build: set -mfpu=neon flag for armv7-a with meson

Building on armv7a with meson currenctly fails:

/usr/lib/gcc/arm-linux-gnueabihf/8/include/arm_neon.h:10369:1: error:
    inlining failed in call to always_inline ‘vld1q_s32’: target
    specific option mismatch

Set -mfpu=neon on that architecture like the legacy makefiles do to fix
the issue.

Fixes: b1d48c41189a ("build: support ARM with meson")
Cc: stable@dpdk.org
Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>