[OPENMP, NVPTX] Support several images in the executable.

Summary:
Currently Cuda plugin supports loading of the single image, though we
may have the executable with the several images, if it has target
regions inside of the dynamically loaded library. Patch allows to load
multiple images.

Reviewers: grokos

Subscribers: guansong, openmp-commits, kkwli0

Differential Revision: https://reviews.llvm.org/D49036

llvm-svn: 336569