powerpc: refactor stpcpy, stpncpy, strcpy, and strncpy IFUNC.

Clean up the IFUNC implementations for powerpc in order to remove
unneeded macro definitions.

Tested on ppc64le with and without --disable-multi-arch flag.

	* sysdeps/powerpc/powerpc64/multiarch/stpcpy-power8.S: Define the
	implementation-specific function name and remove unneeded macros
	definition.
	* sysdeps/powerpc/powerpc64/multiarch/stpncpy-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/stpncpy-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcpy-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncpy-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncpy-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/power7/strncpy.S: Set a default
	function name if not defined.
	* sysdeps/powerpc/powerpc64/power8/strcpy.S: Likewise.
	* sysdeps/powerpc/powerpc64/power8/strncpy.S: Likewise.