The --cpumask flag only takes one single flag name, one can't set
a combination like neon+dotprod.

Therefore, apply the same pattern as for x86, by adding mask values
that contain all the implied lower level flags.

This is somewhat complicated, as the set of features isn't entirely
linear - in particular, SVE doesn't imply either dotprod or i8mm,
and SVE2 only implies dotprod, but not i8mm.

This makes sure that "dav1d --cpumask dotprod" actually uses any
SIMD at all, as it previously only set the dotprod flag but not
neon, which essentially opted out from all SIMD.