reorder 'struct block' fields.

reorder 'struct block' fields.

On 64-bit builds, there was a 32-bit hole near the beginning. Shuffle this
towards the middle of the struct (just before the blk_u union).

Unfortunately this (moved) hole can't be filled by having an I32 at the
start of each union member, since struct block_eval is already the largest
and has no 32-bit fields.

Still, moving further on than some of the hotter fields can't hurt.