anv: Add support for the on-disk shader cache

anv: Add support for the on-disk shader cache

The Vulkan API provides a mechanism for applications to cache their own
shaders and manage on-disk pipeline caching themselves.  Generally, this
is what I would recommend to application developers and I've resisted
implementing driver-side transparent caching in the Vulkan driver for a
long time.  However, not all applications do this and, for some
use-cases, it's just not practical.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>