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/memcmp-power4.S: Define the
implementation-specific function name and remove unneeded
macros definition.
* sysdeps/powerpc/powerpc64/multiarch/memcmp-power7.S: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/memmove-power7.S: Likewise.
* sysdeps/powerpc/powerpc64/power4/memcmp.S: Set a default function
name if not defined and pass as parameter to macros accordingly.
* sysdeps/powerpc/powerpc64/power7/memcmp.S: Likewise.
* sysdeps/powerpc/powerpc64/power7/memmove.S: Likewise.
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