sd-dlopen: introduce SD_ELF_NOTE_DLOPEN_ANCHORED() macro

sd-dlopen: introduce SD_ELF_NOTE_DLOPEN_ANCHORED() macro

SD_ELF_NOTE_DLOPEN_ANCHORED() emits a .note.dlopen ELF note that is
"anchored" to a dummy symbol via the SHF_LINK_ORDER ('o') section flag,
in addition to SHF_GROUP ('G') for folding identical notes together.

Unlike the plain SD_ELF_NOTE_DLOPEN() macro, this variant ties the
note's lifetime to a dummy symbol named with the specified tag: if the
linker's --gc-sections removes the function that calls the macro (e.g.
because the function is never referenced), the associated .note.dlopen
entry is garbage-collected along with it.

Currently, the new macro is unused in our code, but all dlopen notes
will be generated with this in later commits.