[Driver][OpenMP] Build jobs for OpenMP offloading actions for targets using gcc tool chains.

Summary:
This patch adds logic to create jobs for OpenMP offloading actions by:
 - tuning the jobs result information to use the offloading prefix even for (device) linking actions.
 - replacing the device inputs of the host linking jobs by a linker script that embed them in the right sections.

Reviewers: echristo, tra, jlebar, rsmith, ABataev, hfinkel

Subscribers: mkuron, whchung, mehdi_amini, cfe-commits, Hahnfeld, andreybokhanko, arpith-jacob, carlo.bertolli, caomhin

Differential Revision: https://reviews.llvm.org/D21847

llvm-svn: 285319