Partial backport. Only the i686 piece is needed in glibc 2.17, and
the x86_64 piece doesn't apply anyway because we lack the multiarch
implementations present in upstream. Instead the the x86_64 piece
is fixed by glibc-rh1447808-2.patch. 

commit e826574c985a15a500262f2fbd21c7e9259d3d11 (origin/master, origin/HEAD)
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Jun 12 15:00:33 2018 +0200

    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>

Index: glibc-2.17-c758a686/sysdeps/i386/i686/multiarch/strncmp-c.c
===================================================================
--- glibc-2.17-c758a686.orig/sysdeps/i386/i686/multiarch/strncmp-c.c
+++ glibc-2.17-c758a686/sysdeps/i386/i686/multiarch/strncmp-c.c
@@ -1,4 +1,4 @@
-#ifdef SHARED
+#if defined (SHARED) && IS_IN (libc)
 # define STRNCMP __strncmp_ia32
 # undef libc_hidden_builtin_def
 # define libc_hidden_builtin_def(name)  \