libc/glibc: fix downloading addons

Do to glibc what we did to eglibc in #dff359adf15c.

Only (very) old versions of glibc have other external addons,
and they are no longer meaningful.

But for consistency, do the change nonetheless.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 reporter_name="Esben Haabendal"

 reporter_url="esben@haabendal.dk"

 reporter_comment="Example for building a toolchain for m68k no-mmu Linux targets"