[OpenMP] Error when trying to offload to an unsupported architecture

Summary: Throw an error when offloading is unsupported for a particular target architecture.

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

Reviewed By: ABataev

Subscribers: cfe-commits, rengolin

Differential Revision: https://reviews.llvm.org/D32035

llvm-svn: 310307