[coreboot] System gcc requirements

I recently looked in to why I couldn't build coreboot on one of my systems any longer and I think I found the cause. It looks like vboot uses features not available in gcc 6 on Debian Stretch. I actually did manage to get it to build and work by commenting out the offending gcc warning flags and a fallthrough switch attribute, but that's kind of pointless. Is there an expected minimal system gcc version and if so, is it documented? I couldn't find it noted anywhere. Branden