sys-libs/glibc: switch -fstack-protector-all to -strong, bug #712356

sys-libs/glibc: switch -fstack-protector-all to -strong, bug #712356

--enable-stack-protector=all adds protection to functions that don't
have a chance to corrupt stack. On top of that glibc's own tests fail
on static IFUNC resolver.

Let's use -fstack-protector-strong which matches Gentoo's gcc default.

Should fix at least the following test failures:
    FAIL: elf/ifuncmain9picstatic
    FAIL: elf/ifuncmain9static

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/712356
Bug: https://sourceware.org/PR25680
Package-Manager: Portage-2.3.94, Repoman-2.3.20
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>