Blender Git Loki

Cycles: Use pre-compiled PTX kernel for older generation when no matching one is found

This patch changes the discovery of pre-compiled kernels, to look for any PTX, even if
it does not match the current architecture version exactly. It works because the driver can
JIT-compile PTX generated for architectures less than or equal to the current one.
This e.g. makes it possible to render on a new GPU architecture even if no pre-compiled
binary kernel was distributed for it as part of the Blender installation.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8332