[libomptarget] Also support several images for elf

In revision r336569 (D49036) libomptarget support for multiple nvidia images
has been fixed in case a target region resides inside one or multiple
libraries and in the compiled application. But the issues is still present
for elf images.
This fix will also support multiple images for elf.

Patch by Jannis Klinkenberg

Reviewers: protze.joachim, ABataev, grokos

Reviewed By: protze.joachim, ABataev, grokos

Subscribers: openmp-commits

Differential Revision: https://reviews.llvm.org/D49418

llvm-svn: 337355