powerpc: refactor memcmp and memmove IFUNC.

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