v3dv/pipeline: support for specialization constants

v3dv/pipeline: support for specialization constants

That it is justconvert VkSpecializationInfo to
nir_spirv_specialization and pass it to spirv_to_nir.

The code is also basically the same used by anv, tu, and radv
Eventually it would make sense to move it to a common place.

Note that we are using calloc there to allocate the temporary
spec_entries. Trying to use vk_alloc2 causes some problems with the
nir_validate.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>