mk: require SSE4.2 support on all x86 platforms

mk: require SSE4.2 support on all x86 platforms

Increase the default baseline from "core2" architecture to "corei7". This
means that all builds will have SSE4.2 support included, and we can remove
special case manipulation of CFLAGS for the same. Naturally, this does mean
that some machines that previously could run DPDK now can't do so, but
hardware with SSE4.2 has been around for almost a decade now, so this
should not be a major problem.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>