libc/glibc: commonalise extraction btw glibc and eglibc

glibc and eglibc have a very similar extraction process, so it
makes sense to commonalise it.

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=""