[lld][MachO] Fix ObjC stubs from autolinked archives
Move ObjC stub preparation after LC_LINKER_OPTION processing so archive
members loaded through autolink can contribute `_objc_msgSend$` selector stubs
before selector references are finalized.
Otherwise, stubs discovered from autolinked archives may not get matching
`__objc_methname` and `__objc_selrefs` entries needed during stub emission.