x86: Make strncmp usable from rtld

Due to the way the conditions were written, the rtld build of strncmp
ended up with no definition of the strncmp symbol at all: The
implementations were renamed for use within an IFUNC resolver, but the
IFUNC resolver itself was missing (because rtld does not use IFUNCs).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit e826574c)