src: simplify alignment-handling code

Use a common function to handle alignment computations in
multiple places.

PR-URL: https://github.com/nodejs/node/pull/33884


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>