[OpenMP] Consistently use cubin extension for nvlink

This was previously done in some places, but for example not for
bundling so that single object compilation with -c failed. In
addition cubin was used for all file types during unbundling which
is incorrect for assembly files that are passed to ptxas.
Tighten up the tests so that we can't regress in that area.

Differential Revision: https://reviews.llvm.org/D40250

llvm-svn: 318763