build: use default flags for default Arm machine

build: use default flags for default Arm machine

When building for generic distribution we need a stable baseline
architecture, or depending on the build worker the result will vary.

Force the default flags if the user explicitly sets machine=default
at configuration time.

Fixes: b1d48c41189a ("build: support ARM with meson")
Cc: stable@dpdk.org
Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Pavan Nikhilesh <pbhagavatula@marvell.com>