[Clang][OpenMP][NVPTX] Replace `libomptarget-nvptx-path` with `libomptarget-nvptx-bc-path`

D94700 removed the static library so we no longer need to pass
`-llibomptarget-nvptx` to `nvlink`. Since the bitcode library is the only device
runtime for now, instead of emitting a warning when it is not found, an error
should be raised. We also set a new option `libomptarget-nvptx-bc-path` to let
user choose which bitcode library is being used.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D95161