cell: allocate batch buffers w/ 16-byte alignment

cell: allocate batch buffers w/ 16-byte alignment

Replace cell_batch{align,alloc)*() with cell_batch_alloc16(), allocating
multiples of 16 bytes that are 16 byte aligned.

Opcodes are stored in preferred slot of SPU machine word.

Various structures are explicitly padded to 16 byte multiples.

Added STATIC_ASSERT().