[OpenMP] Add flag for overwriting default PTX version for OpenMP targets

Summary:
This flag "--fopenmp-ptx=" enables the overwriting of the default PTX version used for GPU offloaded OpenMP target regions: "+ptx42".



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

Reviewed By: ABataev

Subscribers: rengolin, cfe-commits

Differential Revision: https://reviews.llvm.org/D29660

llvm-svn: 310489