fix compiling warnings with GCC

fix compiling warnings with GCC

GCC 7 plus Ubuntu-16.04 default ccache 3.2.4 reports warnings on switch case fall-through code,
which is commonly seen in DPDK code, which will cause image building failure finally.
It requires newer ccache version to support -Wimplicit-fallthrough.
To suppress the warning, if GCC is version 7 or higher, and it's old ccache version,
will disable the fall-through check.

dpdk-18.05/drivers/net/ark/ark_ethdev_rx.h:39:0,
                  from dpdk-18.05/drivers/net/ark/ark_ethdev_rx.c:36:
dpdk-18.05/arm64-armv8a-linuxapp-gcc/include/rte_mbuf.h: In function ‘rte_pktmbuf_alloc_bulk’:
dpdk-18.05/arm64-armv8a-linuxapp-gcc/include/rte_mbuf.h:1292:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
     idx++;
     ~~~^~

Change-Id: I4d12492471fadef9d349ba9e853a6799512f76f5
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>