The recent slab alignment changes broke an unknown number of architectures
(parisc and x86_64 for sure) by causing task_structs to be insufficiently
aligned.

We need good alignemnt because architectures do things like dumping FP state
into the task_struct with instructions which require particular alignment (I
think).

So change the default alignment to L1_CACHE_BYTES, which is what we used to
have, via SLAB_HW_CACHE_ALIGN.