libc/glibc: no default mode for backend

The caller SHALL explicitly ask for a nmode, and not rely on a default mode.
That's what actually happens, so we can get rid of the default.

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

 reporter_name="Gustavo Zacarias <gustavo@zacarias.com.ar>"

 reporter_url="http://sourceware.org/ml/crossgcc/2011-03/msg00028.html"

 reporter_comment=""