[OpenMP] Fix passing of -m arguments to device toolchain

AuxTriple is not set if host and device share a toolchain. Also,
removing an argument modifies the DAL which needs to be returned
for future use.
(Move tests back to offload-openmp.c as they are not related to GPUs.)

Differential Revision: https://reviews.llvm.org/D38258

llvm-svn: 314329