[llvm] [Flang-RT] Set LD_LIBRARY_PATH for unittests (PR #170840)

[Kelvin Li via llvm-commits]

kkwli wrote:

> What is the error on Linux (Power) with only `libflang_rt.runtime.a` being built? It cannot be related to LD_LIBRARY_PATH since it had no other choice than linking the static libary (if that failed then there wouldn't even be a `ISO-Fortran-binding.test`/`RuntimeTests` file).

```
FAIL: flang-rt-OldUnit :: Evaluate/reshape.test (1 of 254)        
******************** TEST 'flang-rt-OldUnit :: Evaluate/reshape.test' FAILED ********************                                                       
/scratch/kli/wrk/f/build-flang-rt-local/flang-rt/unittests/Evaluate/reshape.test: error while loading shared libraries: libflang_rt.runtime.so: cannot open shared object file: No such file or directory                                                                                                                           
********************                                                                  
FAIL: flang-rt-OldUnit :: Evaluate/ISO-Fortran-binding.test (2 of 254)                
******************** TEST 'flang-rt-OldUnit :: Evaluate/ISO-Fortran-binding.test' FAILED ********************                                           
/scratch/kli/wrk/f/build-flang-rt-local/flang-rt/unittests/Evaluate/ISO-Fortran-binding.test: error while loading shared libraries: libflang_rt.runtime.so: cannot open shared object file: No such file or directory
********************
```

> 
> What CMake options are you using?

```
cmake -G Ninja $HOME/llvm-project/runtimes -DCMAKE_BUILD_TYPE=Release \
 -DCMAKE_CXX_STANDARD=17 -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DLLVM_ENABLE_ASSERTIONS=ON \
 -DLLVM_TARGETS_TO_BUILD="PowerPC" -DCMAKE_INSTALL_PREFIX=$PWD/install \
 -DLLVM_LIT_ARGS="--threads=80 -v --show-unsupported" \
 -DCMAKE_CXX_COMPILER=$HOME/install/bin/clang++ \
 -DCMAKE_C_COMPILER=$HOME/install/bin/clang -DLLVM_ENABLE_RUNTIMES=flang-rt \
 -DCMAKE_Fortran_COMPILER=$HOME/install/bin/flang \
 -DLLVM_LIT_ARGS="-v --show-unsupported" -Wno-dev \
 -DLLVM_EXTERNAL_LIT=./llvm-lit -DFLANG_RT_ENABLE_SHARED=ON 
```

https://github.com/llvm/llvm-project/pull/170840