kernel: k_aligned_alloc: simplify assertion

Let's avoid division and modulus operations as they're costly... and even
more so when they're unnecessary as in this case the main constraint is
about the alignment being a power of 2 which is a very small subset of
sizeof(void *) multiples.

Then move the assertion to common code for wider coverage.

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