Remove duplicate strcat implementations

Since commit 6e46de42 default strcat implementation is essentially
the same for specialized ia64 and powerpc ones.  This patch removes the
redundant implementation and adjust powerpc64 ifunc code to use the
default one.

Checked on powerpc32-linux-gnu (default and power4) and ia64-linux build
and on powerpc64le-linux-gnu.

	* sysdeps/ia64/strcat.c: Remove file.
	* sysdeps/powerpc/strcat.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcat-power7.c: Use default
	C implementation.
	* sysdeps/powerpc/powerpc64/multiarch/strcat-power8.c: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcat-ppc64.c: Likewise.