[OpenMP] Add flag for linking runtime bitcode library

Summary: This patch adds an additional flag to the OpenMP device offloading toolchain to link in the runtime library bitcode.

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

Reviewed By: ABataev, grokos

Subscribers: jholewinski, guansong, cfe-commits

Differential Revision: https://reviews.llvm.org/D43197

llvm-svn: 327460