vtn: add an option to create a nir library from spirv

This adds an options to turn a spir-v library into a set of nir
functions. There is no entry point and all function implentations
are emitted.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6035>
2 files changed