[PATCH v3] Add random memcpy test

[Wilco Dijkstra]

Hi Steve,
    
> It looks like the Makefile change got pushed out but the
> new benchtests/bench-memcpy-random.c file did not.

Yes it seems I forgot to do a git add of the actual file... Sorry about that.

Since I'm on holiday with no access to my GLIBC tree, it's probably best
to revert the Makefile change for now and I'll redo it when I get back.

Cheers,
Wilco