broadcom/compiler: add a couple of shader key helpers

broadcom/compiler: add a couple of shader key helpers

Our shader key includes a void pointer that we can't just memcmp,
so add helpers that allow us toget the 'static' portion and size
of a key. We will use this to fix up the shader cache in v3d in
a later patch.

Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25418>