CFLAGS: Use -fstack-protector-all over -strong

CFLAGS: Use -fstack-protector-all over -strong

Although -fstack-protector-strong will omit inserting any canary checks
on functions that cannot possibly overflow their stack, we will need all
functions to be protected in order to run our check.

It is more benefitial to us to have the check than optimise for
performance. We also expect LTO to inline all those functions that
possibly do not have a stack.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>