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.