powerpc: Create a powerpc64 config feature

powerpc: Create a powerpc64 config feature

The config symbol TARGET_ps3 is used in the build files where
special handling for the 64 bit PowerPC processors is needed.
Introduce a new config feature powerpc64, and replace the use
of TARGET_ps3 with powerpc64.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
SVN-Revision: 17306