Do not stack-protect ifunc resolvers [BZ #7065]

When dynamically linking, ifunc resolvers are called before TLS is
initialized, so they cannot be safely stack-protected.

We avoid disabling stack-protection on large numbers of files by
using __attribute__ ((__optimize__ ("-fno-stack-protector")))
to turn it off just for the resolvers themselves.  (We provide
the attribute even when statically linking, because we will later
use it elsewhere too.)