powerpc: refactor memcpy and mempcpy 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/memcpy-a2.S: Define the
	implementation-specific function name and remove unneeded
	macros definition.
	* sysdeps/powerpc/powerpc64/multiarch/memcpy-cell.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memcpy-power4.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memcpy-power6.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memcpy-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/memcpy-ppc64.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/mempcpy-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/a2/memcpy.S: Set a default function
	name if not defined and pass as parameter to macros accordingly.
	* sysdeps/powerpc/powerpc64/cell/memcpy.S: Likewise.
	* sysdeps/powerpc/powerpc64/memcpy.S: Likewise.
	* sysdeps/powerpc/powerpc64/power4/memcpy.S: Likewise.
	* sysdeps/powerpc/powerpc64/power6/memcpy.S: Likewise.
	* sysdeps/powerpc/powerpc64/power7/memcpy.S: Likewise.
	* sysdeps/powerpc/powerpc64/power7/mempcpy.S: Likewise.