lowlevel-blt-bench: add CSV output mode

Add a command line option for choosing CSV output mode.

In CSV mode, only the results in Mpixels/s are printed in an easily
machine-parseable format. All user-friendly printing is suppressed.

This is intended for cases where you benchmark one particular operation
at a time. Running the "all" set of benchmarks will print just fine, but
you may have trouble matching rows to operations as you have to look at
the tests_tbl[] to see what row is which.

Reviewed-by: Ben Avison <bavison@riscosopen.org>

v2: don't add a space after comma in CSV.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>