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