ppc64: default build as the merged 'powerpc' architecture

After the last merge of the new unified 'powerpc' architecture, ppc64 no
longer compiles cleanly as a standalone architecture.  Some bits and
pieces still exist as files under the old ppc64 hierarchy, but the old
"ARCH=ppc64" is dead.

So if "uname" says ppc64, that now implies that the default architecture
should be "powerpc".

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>