libc/glibc: add option to force unwind

We make it an option, as not all combinations of architectures
vs. compiler vs. glibc/eglibc exhibit the issue. Mostly visible
on old glibc versions, it seems...

This is a missing part from the glibc/eglibc merger... :-/

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

 reporter_name="Yann E. MORIN"

 reporter_url=""

 reporter_comment="EXPERIMENTAL uClibc/NPTL snapshot."