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.
