[PowerPC64] Wrap str{,n}cmp-power{8,9}.S in IS_IN(libc)

These functions aren't used in ld.so at the moment since we don't have
strcmp or strncmp ifuncs for them there.  Remove the ld.so bloat.

	* sysdeps/powerpc/powerpc64/multiarch/strcmp-power8.S: Wrap in
	IS_IN (libc).
	* sysdeps/powerpc/powerpc64/multiarch/strcmp-power9.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-power9.S: Likewise.