k_heap_aligned_alloc: validate the alignment argument

There is a special internal understanding between `z_alloc_helper()`
and `sys_heap_aligned_alloc()` for the meaning of non-power-of-two
alignment values. There was a time when `z_alloc_helper()` was expressed
in terms of `k_heap_aligned_alloc()` so the later had to accept special
alignment values from the former. This is no longer the case and
`k_heap_aligned_alloc()` should enforce proper alignment values now.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>