eal: fix build with intrinsics and gcc < 4.8
eal: fix build with intrinsics and gcc < 4.8

Provide a fallback if the intrinsic __builtin_bswap16 is not available.

Signed-off-by: Intel

mixja/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,msune/dpdk,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,mixja/dpdk,mixja/dpdk,mixja/dpdk,msune/dpdk