Pre-RFC: stabilization of target_feature

[parched]

@est31

Later on we can also add semi-static branching that runs a cpuid or /proc/cpuinfo check on the first time we visit, and for subsequent runs has a much smaller overhead (in the best case: none) by having self-updated the code its running to hard wire the behaviour to the decision just made, or (maybe smarter to do it this way) by setting a function pointer. There has been discussion on that optimisation already, but I think for the initial stabilisation proposal its too much.

This is actually something I have been working on using a procedural macro attribute, see Suggestion for a low-effort way to take advantage of SIMD and other architecture specific tricks LLVM knows about - #3 by parched