kernel: mempool: decouple simple alloc from aligned_alloc

When k_malloc() is expressed in terms of k_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.

Also bypass k_heap_malloc() and friends given they're invoked with
K_NO_WAIT. Go directly to sys_heap_*() instead to cut some more unneeded
overhead.

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