powerpc: refactor strcasecmp, strcmp, and strncmp 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/strcasecmp_l-power7.S: Define
	the implementation-specific function name and remove unneeded
	macros definition.
	* sysdeps/powerpc/powerpc64/multiarch/strcmp-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcmp-power8.S Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcmp-power9.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strcmp-ppc64.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-power4.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-power7.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-power8.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-power9.S: Likewise.
	* sysdeps/powerpc/powerpc64/multiarch/strncmp-ppc64.S: Likewise.
	* sysdeps/powerpc/powerpc64/power4/strncmp.S: Set a default function
	name if not defined and pass as parameter to macros accordingly.
	* sysdeps/powerpc/powerpc64/power7/strcmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/power7/strncmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/power8/strcmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/power8/strncmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/power9/strcmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/power9/strncmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/strcmp.S: Likewise.
	* sysdeps/powerpc/powerpc64/strncmp.S: Likewise.