lmbench: avoid gcc optimize-away multiplication

Change expression used in do_integer_mul and do_uint64_mul
benchmarks so GCC doesn't optimize-away the loops,

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>