z_heap_aligned_alloc(): avoid memory wastage

The strategy used in z_heap_aligned_alloc() was to allocate an extra
align-sized memory block for storing a pointer to the memory heap.
This is wasteful in terms of memory usage when alignment is larger
than a pointer width. A loop is needed to find the initial memory
start when freeing it which isn't optimal either.

Instead, let's have sys_heap_aligned_alloc() rewind a pointer after
it is aligned to make just enough room for storing our heap reference.
This way the heap reference is always located immediately before the
aligned memory and any unused memory is returned to the heap.

The rewind and alignment values may coincide in which case only
the alignment is necessary anyway.

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