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