Previously it was a tagged union which was one of:
* baseline
* a specific CPU
* a set of features
Now, it's possible to have a CPU but also modify the CPU's feature set
on top of that. This is closer to what LLVM does.
This is more correct because Zig's notion of CPUs (and LLVM's) is not
exact CPU models. For example "skylake" is not one very specific model;
there are several different pieces of hardware that match "skylake" that
have different feature sets enabled.
