[OpenMP] Add flag for disabling the default generation of relocatable OpenMP...

[OpenMP] Add flag for disabling the default generation of relocatable OpenMP target code for NVIDIA GPUs.

Summary: Previously we have added the "-c" flag which gets passed to PTXAS by default to generate relocatable OpenMP target code by default. This set of flags exposes control over this behaviour.

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

Reviewed By: ABataev

Subscribers: Hahnfeld, rengolin, cfe-commits

Differential Revision: https://reviews.llvm.org/D29659

llvm-svn: 310484