>> do I still get the same package ? Yes. cc(1) does not use microarchitecture-specific features unless you provide "-march" explicitly. Other BSDs do it, but OpenBSD does not. So, cc(1) only knows that you are building something for amd64. There should not be any difference between nehalem and coffee lake ( both running openbsd amd64 of course).
Some applications detect AVX extensions dynamically (using cpuid probably), and use them if can. OpenBSD is good example (from 5.8): "Kernel supports x86 AVX instructions on CPUs that have them." You do not need to recompile kernel to get AVX. You just install it, and it uses AVX if CPU provides such instructions. Same with packages.
