bench-slope: add MPI benchmarking

* tests/bench-slope.c (MPI_START_SIZE, MPI_END_SIZE, MPI_STEP_SIZE)
(MPI_NUM_STEPS, bench_mpi_test, mpi_test_names, bench_mpi_mode)
(bench_mpi_hd, bench_mpi_init, bench_mpi_fre, bench_mpi_do_bench)
(mpi_ops, mpi_modes, mpi_bench_one, _mpi_bench, mpi_match_test)
(mpi_bench): New.
(print_help): Add mention of 'mpi'.
(main): Add "mpi" tests.

Patch adds MPI operation benchmarking for bench-slope:

$ tests/bench-slope --cpu-mhz auto mpi
MPI:

|  nanosecs/byte   mebibytes/sec   cycles/byte  auto Mhz

add | 0.054 ns/B 17580 MiB/s 0.298 c/B 5500
sub | 0.083 ns/B 11432 MiB/s 0.459 c/B 5500
rshift3 | 0.033 ns/B 28862 MiB/s 0.182 c/B 5499
lshift3 | 0.093 ns/B 10256 MiB/s 0.511 c/B 5500
rshift65 | 0.096 ns/B 9888 MiB/s 0.530 c/B 5500
lshift65 | 0.093 ns/B 10228 MiB/s 0.513 c/B 5500
mul4 | 0.074 ns/B 12825 MiB/s 0.409 c/B 5500
mul8 | 0.072 ns/B 13313 MiB/s 0.394 c/B 5500
mul16 | 0.148 ns/B 6450 MiB/s 0.813 c/B 5500
mul32 | 0.299 ns/B 3191 MiB/s 1.64 c/B 5500
div4 | 0.458 ns/B 2080 MiB/s 2.52 c/B 5500
div8 | 0.458 ns/B 2084 MiB/s 2.52 c/B 5500
div16 | 0.602 ns/B 1584 MiB/s 3.31 c/B 5500
div32 | 0.926 ns/B 1030 MiB/s 5.09 c/B 5500
mod4 | 0.443 ns/B 2151 MiB/s 2.44 c/B 5500
mod8 | 0.443 ns/B 2152 MiB/s 2.44 c/B 5500
mod16 | 0.600 ns/B 1590 MiB/s 3.30 c/B 5500
mod32 | 0.924 ns/B 1032 MiB/s 5.08 c/B 5500

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>