Add experimental musl support (#38825)

This adds experimental support of building systemd with musl. This
allows to build systemd with musl and run most of the unit tests.
Running integration tests is not yet supported, however. Hopefully, this
should be a good starting point to support musl.

This requires musl-1.2.5 with
https://github.com/kraj/musl/commit/fde29c04adbab9d5b081bf6717b5458188647f1c.
The patch is already backported to Alpine/postmarketOS's musl package
since musl-1.2.5-r11. See
https://gitlab.alpinelinux.org/alpine/aports/-/commit/333e50c2052c74bb372f821fdeea2e6814f2fe7a.