lowlevel-blt-bench: add option to skip memcpy measurement

lowlevel-blt-bench: add option to skip memcpy measurement

The memcpy speed measurement takes several seconds. When you are running
single tests in a harness that iterates dozens or hundreds of times, the
repeated measurements are redundant and take a lot of time. It is also
an open question whether the measured speed changes over long test runs
due to unidentified platform reasons (Raspberry Pi).

Add a command line option to set the reference memcpy speed, skipping
the measuring.

The speed is mainly used to compute how many iterations do run inside
the bench_*() functions, so for repeated testing on the same hardware,
it makes sense to lock that number to a constant.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ben Avison <bavison@riscosopen.org>