[libomptarget]Add a trap for external omptarget from LLVM

Old LLVM installation may expose its internal omptarget CMake target when being used by find_package(LLVM) and caused issues in the CMake of libomptarget that is being built. Trap the issue early.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D128129