anv: Use a dynamic array for storing executables in pipeline

anv: Use a dynamic array for storing executables in pipeline

Avoids waste for pipelines that don't use all the shaders, and is
flexible enough to cover cases where there are multiple variants per
shader (e.g. SIMD8/16/32 for fragment shader).

Even though we could pre-calculate the exact size of the array, this
is not a critical path so it is worth preventing the bug that will
likely happen when new variants are added but not accounted for.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4040>