[OpenMP] offload linking issue

[ye-luo]

Broken since d61d72d

$ clang++ -fopenmp -fopenmp-targets=nvptx64 -O3 complex.cpp 
nvlink error   : Undefined reference to '__kmpc_kernel_parallel' in '/tmp/complex-496a9f-nvptx64-nvidia-cuda-sm_80-4864b4.cubin'
nvlink error   : Undefined reference to '__kmpc_kernel_end_parallel' in '/tmp/complex-496a9f-nvptx64-nvidia-cuda-sm_80-4864b4.cubin'
/soft/llvm/main-patched/bin/clang-linker-wrapper: error: 'nvlink' failed

code
affecting main and 15 release

[llvmbot]

@llvm/issue-subscribers-openmp

[ye-luo]

Keep this open until 15.x picks up the fix (revert).

[tstellar]

/cherry-pick b08369f