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