Clang 13.0.0 MPI linking problems on TOSS3.

[rchen20]

Linking problems on TOSS3 when using llnl-toss3-clangX.cmake:

cmake -C ../host-configs/llnl-toss3-clang13.cmake -DCMAKE_BUILD_TYPE=Release -DENABLE_CHAI=On -DENABLE_RAJA_PLUGIN=On -DDCHAI_ENABLE_RAJA_PLUGIN=On ..
make -j

[ 65%] Linking CXX executable bin/kripke.exe
/usr/tce/packages/cmake/cmake-3.14.5/bin/cmake -E cmake_link_script CMakeFiles/kripke.exe.dir/link.txt --verbose=1
/usr/tce/bin/clang++-13.0.0 -Wall -Wextra -O3 -ffast-math -Wl,-rpath -Wl,/usr/tce/packages/mvapich2/mvapich2-2.3-clang-13.0.0/lib -fopenmp=libomp CMakeFiles/kripke.exe.dir/src/kripke.cpp.o -o bin/kripke.exe lib/libchai.a lib/libkripke.a lib/libchai.a lib/libumpire.a lib/libRAJA.a -ldl
CMakeFiles/kripke.exe.dir/src/kripke.cpp.o: In function usage()': kripke.cpp:(.text+0x45): undefined reference to MPI_Comm_rank'
kripke.cpp:(.text+0x56): undefined reference to `MPI_Comm_size'

To get here, need to set the following variables in the cmake file (see https://llnl-blt.readthedocs.io/en/develop/tutorial/common_hpc_dependencies.html?highlight=blt_mpi_includes#mpi):

MPI_HOME
MPI_C_COMPILER
MPI_CXX_COMPILER
BLT_MPI_INCLUDES
BLT_MPI_LIBRARIES