lib: Unify alignment of allocators

lib: Unify alignment of allocators

Different internal allocators (memory blocks, linpools, and slabs) used
different way to compute alignment. Unify it to use alignment based on
standard max_align_t type.

On x86_64, this does not change alignment of memory blocks and linpools
(both old and new is 16), but it increases alignment of slabs from 8 to
16.

Minor changes by commiter.