[OpenMP] Add flag for specifying the target device architecture for OpenMP device offloading

Summary:
OpenMP has the ability to offload target regions to devices which may have different architectures.

A new -fopenmp-target-arch flag is introduced to specify the device architecture.

In this patch I use the new flag to specify the compute capability of the underlying NVIDIA architecture for the OpenMP offloading CUDA tool chain.

Only a host-offloading test is provided since full device offloading capability will only be available when [[ https://reviews.llvm.org/D29654 | D29654 ]] lands.

Reviewers: hfinkel, Hahnfeld, carlo.bertolli, caomhin, ABataev

Reviewed By: hfinkel

Subscribers: guansong, cfe-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D34784

llvm-svn: 310263