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>

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =