sys-libs/glibc: make stack protection optional, bug #679788

sys-libs/glibc: make stack protection optional, bug #679788

The change adds new USE=ssp to optionally disable protection of
glibc's internals. Before the change protection was
unconditionally enabled.

It's useful to disable ssp to validate if stack protection
causes any damage (glibc bugs like https://sourceware.org/PR24202
or gcc bugs like https://gcc.gnu.org/PR81996).

And also useful to get more predictable binaries like in
https://bugs.gentoo.org/679788.

Reported-by: Agostino Sarubbo
Bug: https://bugs.gentoo.org/679788
Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>