DPDK build: fix l3fwd build and update Meson opts

Fix l3fwd patch which broke because the code got moved.
Rework two Meson options:
* RTE_LIBRTE_I40E_16BYTE_RX_DESC using Meson option c_args instead of
CFLAGS. According to Meson docs, using c_args is the proper way to pass
C compiler arguments and the support for CFLAGS is there for legacy
reasons and not guaranteed to work.
* Add platform=generic which mirrors VPP configuration. This removes the
need to hack Arm CPU and NUMA values as the generic build uses high
enough values.

Change-Id: Ibcb17879bdf26329881819c332c1782b9c713182
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>