[Offload] Rework handling for loading vendor runtimes (#93073)

Summary:
We previously had multiple options for this, this patch replaces them
with `LIBOMPTARGET_DLOPEN_PLUGINS=` to be a list of plugins to
dynamically use. It defaults to everything right now. This ignores the
`host` plugin because the `libffi` dependency is going to be removed
soon hopefully in https://github.com/llvm/llvm-project/pull/91264.