powerpc: Optimized strncat for POWER8

With new optimized strnlen for POWER8 [1], this patch adds
strncat for power8 to make use of optimized strlen and strnlen.
This is faster than POWER7 current implementation for larger strings.

Tested on powerpc64 and powerpc64le.

[1] https://sourceware.org/ml/libc-alpha/2017-03/msg00491.html

	* sysdeps/powerpc/powerpc64/multiarch/Makefile (sysdep_routines): Add
	strncat-power8.
	* sysdeps/powerpc/powerpc64/multiarch/strncat.c (strncat): Add
	__strncat_power8 to ifunc list.
	* sysdeps/powerpc/powerpc64/multiarch/ifunc-impl-list.c
	(strncat): Add __strncat_power8 to list of strncat functions.
	* sysdeps/powerpc/powerpc64/multiarch/strncat-power8.c: New file.