run experiements on motox.  Change opencl_aes to using 1 work-item for
N/16 work-groups to encrypt an N byte array.  Each work item operates
on a uint4, which is 4 * 32-bit integers (i.e. 16 bytes of the input
array).  Previously implementation used multiple work-items (maximum
available) to operate on the same entry... which was wrong.

Also, don't allocate input/output arrays on the stack (since for around
8MB we will not have enough stackspace and will segfault).  Use malloc.