elf: ld.so --help calls _dl_init_paths without a main map [BZ #27577]

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 332421312576bd7095e70589154af99b124dd2d1 ("elf: Always
set l in _dl_init_paths (bug 23462)").

(cherry picked from commit 4e6db99c665d3b82a70a3e218860ef087b1555b4)