[Libomptarget] Make the plugins link as LLVM libraries

Previously we made `libomptarget` link as an LLVM library so we have
access to the LLVM core libraries. After the initial patch stuck we can
now apply the same changes to the plugins. This will allow us to use
LLVM in all of `libomptarget` when we have uses for them. In the future
this should allow us to remove the dependencies on `libelf`, `libffi`,
and `dl`.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D130262