anv: Use library mtime for cache UUID.

anv: Use library mtime for cache UUID.

Inspired by a similar commit for radv.

Rather than recomputing the timestamp on each make invocation, just
fetch it at runtime.

Thus we no longer get the constant rebuild of anv_device.c and the
follow-up libvulkan_intel.so link, when nothing has changed.

I.e. using make && make install is a little bit faster.

v2: Use bool return type (Ken).

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>