util: add disk_cache_get_function_identifier()

This can be used as a drop in replacement for
disk_cache_get_function_timestamp().

Here we use build-id to generate a driver-id rather than build
timestamp if available. This should resolve issues such as
distros using reproducable builds and flatpak not having
real build timestamps.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 83ea8dd9)