[OpenMP] Fix --libomptarget-nvptx-bc-path in tests

After D151324, which landed as 349c0aa, many libomptarget non-LTO
nvptx64 tests fail with errors like:

```
clang: error: bitcode library '/tmp/llvm-project/build/runtimes/runtimes-bins/openmp/libomptarget/libomptarget-nvptx-sm_70.bc' does not exist
```

This patch updates the bc path.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D152462

openmp/libomptarget/test/lit.cfg

@@ -120,7 +120,7 @@ else: # Unices
     if config.cuda_libdir:
         config.test_flags += " -Wl,-rpath," + config.cuda_libdir
     if config.libomptarget_current_target.startswith('nvptx'):
-        config.test_flags += " --libomptarget-nvptx-bc-path=" + config.library_dir
+        config.test_flags += " --libomptarget-nvptx-bc-path=" + config.library_dir + '/DeviceRTL'
     if config.libomptarget_current_target.endswith('-LTO'):
         config.test_flags += " -foffload-lto"
     if config.libomptarget_current_target.endswith('-JIT-LTO') and evaluate_bool_env(