Make building nsort more portable

Work on making compiling SIMD-optimized cumsum & nsort more robust on x86_64 platforms.

This robustness is only compile-time CPU feature detection-oriented (run-time would seem to require either a non-intrinsics approach to assembly or separate C compilation units.). So, it matters if your nim.cfg/build flags have march=native (or whatnot) in them which is kind of a choice outside the package as it relates to binary compatibility/deployment.