config: fix C++ cross compiler for Arm and PPC

config: fix C++ cross compiler for Arm and PPC

Through some mixup all cross-files for ARM and PowerPC platforms were
using C Preprocessor (cpp) instead of GCC (g++).
This caused meson to fail detecting the C++ compiler presence and
therefore disabling some targets (i.e. C++ include file checks).

Fixes: e53a5299d219 ("build: support vendor specific ARM cross builds")
Cc: stable@dpdk.org
Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>