[OpenMP] Add support for '--offload-arch=native' to OpenMP offloading

This patch adds support for '--offload-arch=native' to OpenMP
offloading. This will automatically generate the toolchains required to
fulfil whatever GPUs the user has installed. Getting this to work
requires a bit of a hack. The problem is that we need the ToolChain to
launch its searching program. But we do not yet have that ToolChain
built. I had to temporarily make the ToolChain and also add some logic
to ignore regular warnings & errors.

Depends on D141078

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D141105