libc/glibc: change default of "forced unwind" to 'y'

Virtually all our targets do support stack unwinding. For some of
them, ./configure can't detect that, and the build fails later on.

So, default to force unwind, and let the user disable it if it
breaks in his/her case.

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