[OpenMP] Add `-Xoffload-linker` to forward input to the device linker

We use the clang-linker-wrapper to perform device linking of embedded
offloading object files. This is done by generating those jobs inside of
the linker-wrapper itself. This patch adds an argument in Clang and the
linker-wrapper that allows users to forward input to the device linking
phase. This can either be done for every device linker, or for a
specific target triple. We use the `-Xoffload-linker <arg>` and the
`-Xoffload-linker-<triple> <arg>` syntax to accomplish this.

Reviewed By: markdewing, tra

Differential Revision: https://reviews.llvm.org/D126226