systemd/abuild: update -systemd split func patch

Build abuild from upstream source and only apply the systemd split func patch. The patch was also adjusted to use -systemd instead of -service for subpackages.

I took what was submitted here upstream, and made this one change to it. I wanted to move it here because there were several different versions of it in at least two different abuild forks I found, and we/pmaports might as well be the "source of truth" for it since it's a very simple abuild patch.

Note: I split this patch off from !5315 (merged) to work around https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2389.