[OpenMP] Fixed the test environment when building along with LLVM

Currently all built libraries in OpenMP are anywhere if building along
with LLVM. It is not an issue if we don't execute any test. However, almost all
tests for `libomptarget` fails because in the lit configuration, we only set
`<build_dir>/libomptarget` to `LD_LIBRARY_PATH` and `LIBRARY_PATH`. Since those
libraries are everywhere, `clang` can no longer find `libomptarget.so` or those
deviceRTLs anymore.

In this patch, we set a unified path for all built libraries, no matter whether
it is built along with LLVM or not. In this way, our lit configuration can work
propoerly.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93736