Optimize dlopen ELF notes via anchoring and explicitly embed them into executables...

This PR optimizes the handling of `dlopen` ELF notes and reduces binary
footprints across the tree.

By enabling compiler section-splitting
(`-ffunction-sections`/`-fdata-sections`), the linker can now accurately
garbage-collect unused code and data. To align with this,
`SD_ELF_NOTE_DLOPEN_ANCHORED()` macro is introduced, which ties `dlopen`
notes to their calling functions so that unused notes are automatically
removed by `--gc-sections`.

Additionally, this explicitly embeds required `dlopen` notes into
individual executables to fix a visibility issue where package managers
missed runtime dependencies invoked indirectly through
`libsystemd-shared.so`.

diff --cc src/nspawn/nspawn-oci.c
Simple merge