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