Provide better guidance on use of special x86 instructions

[ezrosent]

frawk makes particular use of AVX2 instructions; with custom routines that use runtime feature detection. In addition to that, frawk compiles with -C native by default, which leads to emitting AVX instructions outside of these hand-coded SIMD routines that check requisite features are enabled at runtime. Using -C native seems to provide nontrivial performance benefits, so it makes sense to keep it as the default for those who cargo install frawk and thereby build from source.

As we make frawk more generally available via binary releases, however, it'll be helpful to develop a setup (e.g. via QEMU or some similar technology) to build and then validate frawk binaries intended for older architectures. This issue tracks that work.