libc/glibc: add multilib-cacpable backend

For mutlilib, the C library must be built once for each variants.
Special care must be taken to put the resulting libraries in
the proper places.

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=""