Merge shared/exec-directory-util.? into basic/unit-def.?

Merge shared/exec-directory-util.? into basic/unit-def.?

Suggested in
https://github.com/systemd/systemd/pull/35892#discussion_r2180322856.

This is a tiny amount of code and does not warrant having a separate file
and spawning a separate instance of the compiler during the build.

Note: it took me a while to confirm that the contents of that table and
function don't end up in libsystemd.so. The issue is that they _are_ present in
it, unless LTO is used. We actually use link_whole[libbasic_static] for
libsystemd, so we end up with all that code there. LTO is needed to clean
that up.