[OpenMP][NVPTX] Use appropriate _CALL_ELF macro when offloading

Summary: When offloading to a device and using the powerpc64le version of the auxiliary triple, the _CALL_ELF macro is not set correctly to 2 resulting in the attempt to include a header that does not exist. This patch fixes this problem.

Reviewers: Hahnfeld, ABataev, caomhin

Reviewed By: Hahnfeld

Subscribers: guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D51312

llvm-svn: 340772