powerpc: refactor memset 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/memset-power4.S: Define the
	implementation-specific function name and remove unneeded macros
	definition.
	* sysdeps/powerpc/powerpc64/multiarch/memset-power6.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memset-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memset-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memset-ppc64.S: Likewise.
	* sysdeps/powerpc/powerpc64/memset.S: Set a default function name if
	not defined and pass as parameter to macros accordingly.
	* sysdeps/powerpc/powerpc64/power4/memset.S: Likewise.
	* sysdeps/powerpc/powerpc64/power6/memset.S: Likewise.
	* sysdeps/powerpc/powerpc64/power7/memset.S: Likewise.
	* sysdeps/powerpc/powerpc64/power8/memset.S: Likewise.