Add MVAPICH to El Capitan system.py

[pearce8]

• /collab/usr/global/tools/mpi/toss_4_x86_64_ib_cray/opt/mvapich-plus-rocm6.4.3-gnu-5.0.0-1/
• may need to add: export MPICH_GPU_SUPPORT_ENABLED=1
• may need to add: export LD_LIBRARY_PATH=/collab/usr/global/tools/mpi/toss_4_x86_64_ib_cray/opt/mvapich-plus-rocm6.4.3-gnu-5.0.0-1/lib:${LD_LIBRARY_PATH}
• run line: flux run -N 8 -n 32 /collab/usr/global/tools/mpi/toss_4_x86_64_ib_cray/opt/mvapich-plus-rocm6.4.3-gnu-5.0.0-1/libexec/osu-micro-benchmarks/mpi/collective/osu_allreduce -d rocm
• Chris C. @cchambreau testing compatibility and performance. Just GNU compilers for now? Please ping with any updates.