radv: enable secure compile support

radv: enable secure compile support

Can be enabled via the environment variable which tells the
driver how many compilation threads are expected to be called,
and therefore how many forked processes the driver should
create.

For example we would expect to call fossilize replay with
something like this:

RADV_SECURE_COMPILE_THREADS=8 ./fossilize-replay --num-threads 8 \
--shader-cache-size 0 --ignore-derived-pipelines pipeline_cache.foz

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>