Added gather benchmark to cross refer to the 'lazy' analogue of summing a vector with many zeroes in regions not owned by THIS rank

MPI_benchmark/compile-command

@@ -1,2 +1,4 @@
 mpicxx  -fsycl halo_mpi.cc -o halo_mpi
 mpicxx -O2 -std=c++11 io_mpi.cc -o io_mpi
+# Frontier: (hipcc via mpicxx wrapper; ACC_HIP is the default in-file)
+mpicxx -O2 -x hip gather_mpi.cc -o gather_mpi -L${ROCM_PATH}/lib -lamdhip64