x86-64: Use IFUNC strncat inside libc.so

Unlike i386, we can call hidden IFUNC functions inside libc.so since
x86-64 PLT is always PIC.

Tested on x86-64.

	* sysdeps/x86_64/multiarch/strncat-c.c (STRNCAT_PRIMARY): Removed.
	Include <string/strncat.c>.
	* sysdeps/x86_64/multiarch/strncat.c (__strncat): New strong
	alias.
	(__GI___strncat): New hidden alias.