[LinkerWrapper] Perform device linking steps in parallel

This patch changes the device linking steps to be performed in parallel
when multiple offloading architectures are being used. We use the LLVM
parallelism support to accomplish this by simply doing each inidividual
device linking job in a single thread. This change required re-parsing
the input arguments as these arguments have internal state that would
not be properly shared between the threads otherwise.

By default, the parallelism uses all threads availible. But this can be
controlled with the `--wrapper-jobs=` option. This was required in a few
tests to ensure the ordering was still deterministic.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D136701