Without this patch, libomptarget tests fail to find libomp.so, whether
I configure openmp as part of LLVM_ENABLE_RUNTIMES or
LLVM_ENABLE_PROJECTS.
In the LLVM_ENABLE_PROJECTS case only, it's already possible to work
around the problem by setting LD_LIBRARY_PATH appropriately before
calling "ninja check-libomptarget". However, there was a time when
that wasn't necessary, and it seems like it shouldn't be.