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

After D151324, which landed as 349c0aac, 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