kernel: kheap: decouple simple alloc from aligned_alloc

When k_heap_alloc() is expressed in terms of k_heap_aligned_alloc()
it invokes a longer aligned allocation code path with an extra runtime
overhead even though no alignment is necessary.

Let's reference and invoke the aligned allocation code path only when an
actual aligned allocation is requested. This opens the possibility for
the linker to garbage-collect the aligning code otherwise.

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