Do not build the CUBIN conversion pass when NVPTX Backend isn't configured

This pass would currently build, but fail to run when this backend isn't
linked in. On the other hand, we'd like it to initialize only the NVPTX
backend, which isn't possible if we continue to build it without the
backend available. Instead of building a broken configuration, let's
skip building the pass entirely.

Differential Revision: https://reviews.llvm.org/D74592