Refine features to include sse4 for popcnt
popcnt is part of sse4, not sse3/ssse3, so add feature detection for sse4 to enable use of this instruciton. sse4 and ppcnt was first available in the Nehalem architecture, Nov 2008 in the first gen of Core i7 and i5 processors. The oldest machines I have access to date from 2011. Also remove :ssse3 because we don't currently define or use any ssse3 instructions. Add a bit of documentation about popcnt. Setup CI to build with a bootstrap file to get these instructions.
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