mk: allow corei7-avx flag with gcc 4.7
mk: allow corei7-avx flag with gcc 4.7

In rte.toolchain-compat.mk, ensure MACHINE_CFLAGS is initialized when
using GCC 4.7. Update the GCC 4.6 test to also include GCC 4.7.

Signed-off-by: Krzysztof Witek <7713ffd7898d3e015d56326250926b36c41faa47@6wind.com>
Signed-off-by: Zijie Pan <96e60a9833a2f3945af4d0b2739f70e32dd540b4@6wind.com>
Acked-by: Ivan Boule <8408c61bea57ac94dd0d4dac44b115be1b288952@6wind.com>
Acked-by: Adrien Mazarguil <9d39ea493ca46c560334f650ac5d4b5ffbcb5cf0@6wind.com>

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