Generalize `iree_uk_cpu_features_list_t` for x86 (#12749)

This is one last prerequisite for x86 ukernels: with x86 we will need to
have sets of features that combine together > 5 features, e.g. just base
AVX512 is actually a combination of 5 individual features
(AVX512{F,BW,CD,VL,DQ}) and is meant to be added on top of AVX2 features
(itself the combination AVX+AVX2+FMA).

So this generalizes `iree_uk_cpu_features_list_t` to:
- Grow more gracefully to several/many features. Technically it's a
heap-backed vector now, but you'll see that the growth logic is very
YAGNI : we grow once from capacity 0 to 64 hardcoded, and never again
:-) So the code is very simple. And it's an implementation detail of
that grow function if we ever need something better.
- Support definding cpu feature sets that extend others (e.g. define
AVX512 extending AVX2)
- Support optionally giving cpu feature sets short names instead of
enumerating all individual features (otherwise, avx512 test log lines
and benchmark names were very verbose and wrapped around multiple
lines).

Moreover, this adds reporting of the first unsupported feature when a
test gets skipped because of that.