Investigate requiring support for newer instruction sets

There may be performance benefit to compiling with flags allowing the compiler to generate code for newer instruction sets like SSE4.1, AVX, CLMUL, and so on.

There will probably be code simplification, as well. See cpu-info.h.

We might consider doing this only at a new major version. It's also worth investigating (a) when the last chips without these features were shipped, and (b) if Xen, VirtualBox, and so on expose that true instruction sets available via /proc/cpuinfo - sometimes these are masked out