string: Disable stack protector for memset in early static initialization

For ports that use the default memset, the compiler might generate early
calls before the stack protector is initialized (for instance, riscv
with -fstack-protector-all on _dl_aux_init).

Checked on riscv64-linux-gnu-rv64imafdc-lp64d.

Reviewed-by: Florian Weimer <fweimer@redhat.com>