[OpenMP] Make OpenMP generated code for the NVIDIA device relocatable by default

Summary: When device offloading is enabled and the device is an NVIDIA GPU, OpenMP target regions must be compiled with relocation enabled by passing the "-c" flag to the PTXAS invocation.

Reviewers: arpith-jacob, caomhin, carlo.bertolli, ABataev, Hahnfeld, jlebar, hfinkel, tstellar

Reviewed By: Hahnfeld

Subscribers: Hahnfeld, rengolin, mkuron, cfe-commits

Differential Revision: https://reviews.llvm.org/D29642

llvm-svn: 310300