elf: ld.so --help calls _dl_init_paths without a main map [BZ #27577]
In this case, use the link map of the dynamic loader itself as a replacement. This is more than just a hack: if we ever support DT_RUNPATH/DT_RPATH for the dynamic loader, reporting it for ld.so --help (without further command line arguments) would be the right thing to do. Fixes commit 33242131 ("elf: Always set l in _dl_init_paths (bug 23462)").
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