benchtests: Remove useless ORIG_SRC in memmove benchmarks

The ORIG_SRC argument is likely a useless relic from the original
correctness tests that are not needed in the benchmarks.  Remove the
argument and use S1 to point to the source to avoid confusion.

        * benchtests/bench-memmove.c (do_one_test): Remove unused
        ORIG_SRC.
        (do_test): Adjust.
        * benchtests/bench-memmove-large.c (do_one_test): Remove unused
        ORIG_SRC.
        (do_test): Adjust.