Fix strncmp in rtld for x86-64 by using the generic implementation.  Not
needed upstream since upstream uses
d8725b1fba769a89ce2c902a2091d01faa946b66 to enable strncmp from
strncmp.S, but this introduces SSE registers into rtld for RHEL7 and
this would be the first instance of such a change. While we have done
everything we can to save/restore such registers (see the xsave work) it
isn't clear if everything is fixed, so to play it safe in RHEL we just
use the C version of strncmp for rtld.

Not needed upstream.

diff --git a/sysdeps/x86_64/multiarch/rtld-strncmp.c b/sysdeps/x86_64/multiarch/rtld-strncmp.c
new file mode 100644
index 0000000000000000..5bb3cfb974b9a95d
--- /dev/null
+++ b/sysdeps/x86_64/multiarch/rtld-strncmp.c
@@ -0,0 +1 @@
+#include <string/strncmp.c>